Well, this depends on the power output of the engine. Is the engine stock? No mods? If so, 1975 engines had horrible hp. A 500cfm may work........ 600 cfm is a normal size for a 350. Unless it has 400+ hp, then you could go bigger, like a 750 cfm. Also depends on the jetting of the Carb.I have recently purchased a 1975 Camaro LT. the car has a SB 350 but has a 2 barrel on it. I would appreciate some input into what size 4 barrel and what manifold may work best in changing this to a 4 barrel.