if both are 1460 model carbs one is 600cfm the other 750cfm.
they are basically the same except the 750 can flow more air.
usually in the same family of carbs they carry the same power valve and pump shooter sizes, sometimes the jets are larger by 1 or 2 sizes. many times from 600-850cfm the valves and jets are all the same and the carb can just flow more air even the ventries are the same.
the reason a smaller carb is crisper is because the intake restriction causes a rise in vacuum the engine produces.more vacuum increases fuel delivery signal from the carb so it can be richer.
the bigger the cfm to a point. is helpful at higher rpms. so if you are cruising around for 2 hours over 3500rpm then the bigger cfm is needed.
sometimes a cam requires more intake cfm.
problems, if you put a carb cfm that is too big for an engine it will always run lean. this is because the vaccum is lower, and the fuel signal is lower that forces you to basically jet up or increase power valve opening vacuum to compensate.
if you put a carb on a engine that is too small it will always run rich. the vaccum will be high and the engine will not be able to swallow enough air forcing you to either open the butterflies too far or requiring drilling holes in the butterflies to maintain idle speed without opening the butterflies exposing the transition circuit.
many people just assume bigger carb = more horsepower 600 is smaller then 750 so 750 means more.
the question is what problems are you having with the 600cfm you have now?
if you have closed chamber heads and a mild cam, 750 would be pushing too big on a 351 V2. the engine will run more rough at idle then after 2000rpms smooth out. you will have a lower vacuum so if you are running accessories that might start to cause issues. clevelands are sort of a weird beast too because they love to have more and more fuel poured down their throat.
clevelands were also designed as a higher rpm motor since it has some roots in nascar. so you have this situation where a cleveland will wake up at high rpms with a larger carb, but from a street car standpoint that isn't a good thing since you spend 99% of the time driving at 25-45mph street light to street light. in the street car case larger carb means worse idle, lower vaccum, worse MPG.
really it depends on what you want the car to be but also what the engine can tolerate.
for me i went from 600cfm to 750cfm in steps. my engine wouldn't run on the 600 properly at all i kept having to open the butterflies too much.
650 was better i had it for a while. but when i went to 750 suddenly the engine ran a lot better, it still needed alot of tuning but was more in the ballpark. i had bought a 670 holley but never tried it out since the 750 seemed to be a good fit. now i have a V4 converted V2 engine with open chamber heads lower compression that causes all kinds of weirdness so my issue was always with not enough air at idle and then too much air later on. i still have that problem and i did try modifying a 650 cfm carb to flow more at idle and it was a disaster. so i compromised with the 750 to take advantage of the idle flow but i had to de tune the top end in various ways to make it work.
personally i would like to have a 600 or even a 500cfm carb but on my engine parts combo it just won't work.
so again what problems are you a having with the 600 you have now, is it just old and you want to try a larger carb? are you just trying to get more HP from your car because you could advance timing and readjust the 600 to make up the difference.