carb ?

Depends on what your max rpm is. If you aren't shifting higher than 6000 rpm the 600 is more than enough for a street motor. You would need to be running the engine up into the 7000-8000 rpm range on a regular basis to need a 750 carb.

There are a number of calculators on the web (do a search on carburetor sizing) that will help you decide on what size carb to use. If your engine had a 100% volumetric efficiency that is on each intake stroke it would completely fill the cylinder then the engine would require about 650cfm for 6000 rpm. A more realistic effiency would be 85-90% which would require 570cfm making the 600 just about perfect.

Just like a cam resisting the temptation to go big will make for a much more friendly engine on the street.