650 or 750 holley

I wouldn't do anything until after you try the 360 with the 600 Holley. I ran one for almost two years on a 360 and the engine made 370 HP with it. I am running a 670 now and have tried a 725 and 750. The 725 and 750 made no more power than the 670 by the butt dyno.

Unless you are building a all out motor that will be turning in excess of 6000 rpm regularly you just don't need a 750 and it won't have the response in the low mid range where you drive on the street that the smaller carb will have.