You've got a decent combo.
You first need to get it running correctly and make sure all the voltages are within specs. EFI is very forgiving,.. yet very intolerable with certain things.
When I first put my son's car together I had some issues that I didnt understand. I finally got the motor running pretty damn good with EFI. The cam lopes nice, the throttle response is perfect. Its pretty squirelly for what it is and I am pretty proud of myself for sticking it out. Got some good advice and got it all working. I still want to swap to a carb,.. but only because of personal choice. Not for lack of understanding this time.
You need to do the same with yours. You dont need a lot of tools to figure it out. You need a cheap little $9 - $15 volt meter. A timing light. A couple screw drivers and a cheap socket wrench kit will do.
The most noticeable effect that I saw when tuning my son's car was resetting the TPS. It has an aftermarket TB on it and when they put the stock TPS sensor on it they never bothered to make sure it was adjusted correctly. The TPS voltage was off and out of spec.
Follow the instructions within these links and then see how the car is running.
Ford Mustang Base Idle Reset
Summarized/Corrected Base Idle Reset Procedure
Ford Fuel Injection » 10-pin Connector Fix
Cleaning the Ford Mustang Throttle Body - Solving your idle problems
Checking, Setting and Bumping up your Mustang's Timing
Cleaning the salt and pepper shakers didnt seem to have much effect on my son's car. I suspect that his were working fine. Resetting and correcting his TPS was like night and day for that car. Felt like it picked up 15 - 20 hp. Much more crisp throttle response, idles much better and will break traction in 1st and 2nd the second you get past half throttle. Dont have to romp on it or dump the clutch at all for it to break traction.
Resetting the timing helped also. I have it at 14.5 now and it seems very happy with that. No pingning and very crisp through the RPM's.
I had an issue where the RPM's were hanging when coming to a stop from cruising and then the idle would hunt/surge up and down a little bit. I replaced a broken O2 sensor and the speedo wasnt working. I bought a new $26 sending unit. Plugged a new speedo cable in and viola. The hanging idle was gone. It needed the input from the VSS.
Your car may and will probably respond differently. You will have to go through it and hunt down what and where the problem is. All the information you need is out here on the net. I dont like hunting down all of these variables and that is why I prefer a carb over EFI. At the stock levels that both yours and my son's engines are in. A carb will more than likely NOT increase performance. Much will depend on the type of carb, tune of the carb, the intake choice and the timing settings,.. all of that will determine the final performance out put. At this level EFI and Carb are on par with each other when comparing performance. EFI gets the nod IMO because it will be more efficient and get better MPG.
The only time I will argue that a carb motor has the advantage over EFI is when very large ported after market heads, large open plane intakes and cams with a lot of duration are used. Large duration cams kill vacuum, 90% of what an EFI setup needs to be streetable comes from vacuum signals. Even a carb will need different tuning when large duration cams are used. A carb motor will tune much easier and make more power under these conditions than EFI will. I can show before and after results from 3 different motors that I have done. Carb came out the winner in all 3. Not saying EFI cant be made to work. But in the end you'll have more money invested in an EFI setup to reach the same goals.
Right now you have a working EFI set up. A carb setup on your heads and shortblock will probably not show any significant performance gains. Nothing that would make swapping worth the effort. The only worth while reason to swap would be if you have defective electronics and sensors that will cost as much or more to fix as it would to swap to a carb. Then it would make sense to swap to a carb.
Your current engine combo is not enough to need 24# or 30# yet. The 19's should be enough for now.
My son's car is only running 19's and its running a little on the rich side. Simply bumping up to 24# injectors does not make the motor any stronger or faster by themselves. The only reason would be if you were experiencing lean conditions where your 19's were hitting 100% duty cycle. I had the injectors in my son's motor professionally cleaned and bench tested before I put the motor together. If your unsure, have yours tested. You can clean them yourself. Just make sure you have some new O-rings in case your existing ones are dry rotting. You'll have a fuel leak if you put the injectors back in with dry o-rings.
Swapping to carb is a choice that should not be based on popular opinion. If you dont understand how to tune a carb set up, you will be in the exact same boat your in now if its not running right. Also realize that you'll play hell selling a carb'd car to anyone under 35 who has never owned anything except EFI.
Hope that helps.
Hurst