Whats the difference ? 4150 question !

Venturi diameter AND throttle blade size are the basis for CFM ratings.

CFM ratings are a poor way to chose a carb size. You can buy a...say a BLP BX4 carb with a 1.350 venturi and 1.687 throttle bore and it’s rated at 650 CFM but in actual practice it will flow the better side of 750 CFM. Take that same 1.350 venturi and use a 1.750 baseplate and the CFM will go up another 50-75 CFM.

If you are using a main body that has/had a choke horn, I’d shelve it and buy a ProForm main body for it and start there.

If you can find one FST has a carb that is under 500 bucks that has all the bells and whistles and it comes in 1.350 and 1.410 venturi. The former is rated at 650 CFM and the latter is rated at 750 CFM.

Finding one may be near impossible.