Carb for 340

750 is not too big for a 340. Especially one with the right cam. It will need the CFM at the upper RPM range. You can run a 340 on a 650 area carb. But it will lose some top end. A race 340 can take 850+. One guy on here, I forget who, has run a dominator 1050 on his 340 at the 1/4 and the engine loved it.

But you mention dyno tuning. Why not run what you got, see what the dyno guy says, see if he's got some in the shop you can play around with and try, see what results you get. There is no "magic" number for CFM.