How many CFM for my 340?

Yes the 750 would let the motor breathe better on top but why not try this / wait until you swap the 3.91 rear end and put an open 1/2 inch spacer and see how it is...have you messed around jetting the carb at all...they are set up close but I would try 1 or 2 steps richer and see if its any better..is it an AVS carb now, if so lighten up the secondary spring...stock is 2 1/2 turns, try 2 turns and if no bog in high gear down low when floored then try 1 3/4 turns.

The 750 AVS will bring the best ets over a 650 carb but you may find those 3.91 gears wake the motor up nice...and those loose gears will allow the secondaries to open quicker..even now with your 3.58 gears the secondaries are likley not opening as fast as they could.