With regard to the gearing, yeah, lower rpms is better. But there is a point where you have gone too low. There is a lot of air that you have to push our of your way at 60 mph. So it is going to depend on the aerodynamics of the car.
Is there anyway for you to find out what your drag coefficients are? Might be able to find that information on-line.
From there, you would need to know how much horse power (well, torque) you are going to need to keep your car moving at that certain speed. Then you'll need to add at least 10 hp to that to compensate for other variables, like tire pressure, wind direction, things like that. Some would probably even say to add 15-20 to your minimum so you can compensate for extra things like, rain. rain is not just heavy, but it also adds extra pressure you have to push through.
Then, once you have that minimum number and you are happy you've covered all of your bases, you'll want to compare that number to your dyno chart and see at what RPM the engine makes that power at.
I think we broke it down a couple of years ago to figure out that a lowered 240sx needs 17 whp to maintain 65 mph assuming proper tire pressure, and flat surface etc etc. So to compensate for the variables of daily life, I would safely say that you need a minimum of 25 whp to maintain speed.
So for optimum fuel economy, you would have to figure out at what RPM the engine can make 25 whp at a minimal amount of throttle. Sure, 25 whp is like 900 rpms, but that is at WOT. I'm sure it would need to be more like 1600-1800 at 10-15% throttle.
You could get really crazy and add in the duty cycle for the injectors and all of that to determine at what throttle position itself would be the most efficient.
I have no doubt there is a way to calculate all of that. There is a way to calculate how much power you need to push through the air, so the rest of it is just further calculation. But, most of us don't know how to do it, so we would resort to tangible data. Make a few runs on a dyno. One at WOT (cause it is fun and gives you a base to work from) then do a run at 50% throttle. You'll need to rig something up to that it is a true 50%, not a "best guess" Then do another run at 20% throttle. record all of your data and see how different the HP curves are with different throttle positions. The injectors and duty cycles will determine what is better, lower RPMS with more throttle or less throttle at higher RPMS.
Break it down to see how much fuel you'll use over one hour. IE each time the injector sprays, it is this much fuel. times how many injector sprays there are over a second. how many seconds over an hour gives you X amount of fuel used.
Find the point on the hp curve where you make the minimal HP you need to maintain speed. If you set your ratios to that you are cruising at that RPM, you'll need that much throttle to maintain speed.

The actual ratio itself would be of little concern at this point because you are dealing with actual Wheel horse power, and not the power lost through the drive train. A 1:1 ratio has the least amount of power lost, but at that point, you're engine speed would be higher than it needed to be.
But perhaps you could work it out so that you were making the optimum WHP, while maintaining a 1:1 ratio with a minimal amount of throttle. Again, you'd have to break down the math to see how much fuel you would be using then to see if it is any different/better than another option.
Personally, I think you would actually be using more fuel because the engine would be spinning faster, but that is just speculation.

Sorry that was so rambling. might have to go back and clean it up if it doesn't make sense.

In regard to adding that cam to the mix, you'll be hurting fuel economy for the reasons that Loren said. The "better" overlap, just sucks some extra fuel through the cylinder so that you have an entirely fresh mix, instead of the half fresh, half burnt mix like a stock cam provides.

But, since we are rambling on anyway, perhaps there is a way to take advantage of that without giving too much of the power gain.
See, on a stock car, there is an EGR valve that is supposed to suck exhaust fumes back into the intake manifold to be burnt again, reducing the amount of harmful crap that goes out the tail pipe. Usually, by the time someone adds a cam, they have already gotten rid of the EGR valve by installing aftermarket headers. When it comes to making power, particularly NA power, the sum of the parts is greater than the individual totals. IE, an intake will get you 6 hp and an exhaust will get you 6hp, but together, they give you 15 hp. So lets assume we have an intake set up. And an exhaust set up with a nice header the whole bit. And now we want to add in the cam. We know we are already running rich because we are relying on a very basic O2 sensor to compensate for the added flow from the intake and exhaust and we all know how well that works. So going a step further, we add the cam. Notable power gains. Everything is great, but because of the overlap, that is making all of this extra power, our gas mileage is dropped. So we take it to the shop to get tuned. That helps a lot. Now we aren't spraying so much fuel, the timing is a bit better, we have better duty cycle on the injectors, and our air to fuel ratio is closer to the ideal 14.whatever to 1 Awesome.
But, do we think we can get even more MPG?
Going back to the EGR valve, on a stock engine, they designed the exhaust to have a fair amount of back pressure to optimize the EGR function. We've gotten rid of that back pressure so we could make more power. Even though we are tuned, there is still a fair amount of un burnt fuel that is being passed through the exhaust. We could lean the tune out even further, but then we are running the risk of blowing up. But what about pulling that exhaust back into the intake to burn it again, much like the stock EGR system worked?
We would need a strong vacuum source. We would want a good place to pull the exhaust from. We would want to keep the unburnt gas consistent between all the cylinders. Three key points the stock EGR set up just didn't do.
So, a strong vacuum source, that is also going to keep it decently consistent. What about a custom bung just behind the throttle plate? Just needs to be somewhere that is going to feed all of the cylinders evenly. We don't want to feed just one cylinder the crap exhaust, that would cause power balance problems, a tuning nightmare, and a major hot spot. Make it a pretty good size. Maybe the size of the brake booster line. Remember, we don't have any back pressure to assist with this, so the only air we are going to get is whatever we can suck in. (but what about idle? we'll talk about that in a minute) On the exhaust, it is going to have to be after to the collector on the header. The collector part of the header is where a lot of the magic happens as far as to why the header makes power, so we don't want to mess that up, so get at least a few inches behind that to tap into. this should let us get as much as we can from all of the cylinders, and not screw up the flow enough to kill off too much HP.
So now we are capturing that flammable exhaust gas, and burning it again, but because we are tuned, and running well, it isn't like the stock system where we are just picking up soot.
The stock system typically doesn't open until high way cruising speeds. The idea is that it will hurt HP a little bit, and it would kill the idle at low speeds. So maybe install a rpm switch that opens the tube at an acceptable RPM, which you would have to determine on the dyno and it would need to be "tuned for" The hot air, and the un burnt fumes would throw off your whole mix as far as the "perfect" tune goes.
You might be able to get away with keeping it open all of the timing and just compensating for the idle with the idle adjuster, but I doubt there would be enough exhaust fumes in the pipe to warrant it at idle, unless you have an amazing tuner that can compensate for equivalent of a rather large vacuum leak.
Again, it would come back to the tune if it would be worth having it open under acceleration. It might kill off enough hp to negate and of the gains. It might not. Under load, you're spraying more fuel through each cycle, so technically, that is when the exhaust is the worst. If you can get it tuned correctly, then you could have it be just a simple RPM switch to open the valve anytime the rpms are over 1500 and then close again under 1500. You would be able to simply unplug it when you had the "go fast" bug which would keep the valve closed and should let the engine make more power as the bolt ons allow.

again, all speculation based solely on my understanding of the way things work.
I'd like to temper all of this with saying that I have had a long few days so my brain could be a bit fried and I could have forgotten a mucho importante detail
that would alter all of that.