Vikings guess should be right on target. I do think the 240 is closer than the 280. The head you have ,or something very close to that would have would have come on 283---- 220hp and 327---250hp. Neither are close to being as good as the 462 heads on a 66---327---275 hp. CARBURATORS ! If a 1965 Corvette with a 327---350 hp. had a Holley carb. with less than 600 cfm. then its hard to believe that a 750 could possibly help your motor.What do You think?