With the 750 you're probably washing you cylinder walls down with gas. For that engine, a 500 cfm would do just fine. A 600 would also work, but still more than you need.

My 350 has 10.5:1 compression, 2.02 fuelie heads, a Torker I intake, sanderson headers, crane cam and pumps out 350 hp using a 600 cfm. So I would doubt that you would need a 750.