carb size

You are using the big port manifold on the small port heads, thats the first problem. Find a small port 4 bbl intake-I like offys, they come in both port sizes then try to jet the carb if it still seems rich. Yes most will say a 750 is too big and you might get better milage with a 600 but it can work good. Make sure you have a good strong spark, I like my msd 7al box but crane and others have good ones too. Headers are a must also if you want to keep that 750. Its a vacuum carb so thats a plus, I run a holley hp 750 DP on my 318 with 3.91 gears and its fine, plugs stay nice and clean, it has great power at high rpms but the gas milage stinks and I know it would be much better with a eddy 600 but if I want great milage I drive my buick lacrosse car.