A 600 would be pretty much a perfect match for a 5.0. The 85 GT used a 580 CFM Holley,.. so a 600 is right on target. Yours may need to be fine tuned a little bit. Perhaps jetted a little richer or leaner depending on the condition. The 600 on my 306 had a bad off idle stumble when it was new out of the box. Turned out to be a lean stumble. Swapping out the power valve, going to a larger squirter and jetting it up pretty much cured it. But each car is different. Heads, cam, compression ratio, vacume signals,.. all have different effects on a carb. And even the carbs themselve can be different from one to another. My father inlaw built 2 305 Chevy motors for 2 S10 pick up conversions we were building to make a couple bucks on. The first motor ran great with a 600 vacume secondary holley. Tweaked the idle mixure a little bit and that was it,.. perfect right out of the box. But on the other motor,.. same as the first and using an identical carb needed the spring for the secondaries tweaked and needed to be jetted down on the secondaries. Took us a week to get it right. Just one of those things.

I tried running an EFI pump with a single return style regulator and had nothing but problems with it. At idle it would hold 6 PSI of fuel pressure just fine,. but as soon as you got on it a little bit the fuel pressure would fall off to nothing and the bowls would empty out. I tried upping the fuel pressure and anything over 7 PSI and the carb would start spitting fuel out of the top. But again,.. as soon as you got on it.. the fuel pressure would fall off.

I never tried Johns method of using 2 regulators. I recall reading about that somewhere else as well. I found it more reliable to swap out the pumps. A PITA to do,.. but less headaches in the long run.

Hurst