NEVER put a 750 on a 318........

All 4V carbs are tested at 20.4 inches of water. IIRC that is 3 inches of Mercury. 2V carbs are tested at about 10 inches of water which is 1.5 inches of Mercury.

You can 850 CFM through a 250 CFM rated carb by running the test pressure up.

The biggest mistake in running a bigger carb than the math calls for is not getting the booster correct. You can run a much bigger carb if the booster is carefully selected and not lose a single HP down low and gain from the middle all the way up.
Actually it's the other way around, but 2V-3.0" & 4V-1.5" are typical, conversion is sq.root of 2; 1.4142. Going "too big" just presents more tuning challenges, but will outperform too little if You've got it right for sure.