Carb question stock 340

I wouldda left that 750DP on there. They can easily be adjusted to not suck gas so bad. I wouldda spent the coin on Stall and gears instead.
The size of the carb between 500 to 750 has very little to do with how much gas it sucks. That is mostly; how rich the low-speed circuit is, how the accelerator pump is adjusted, the ignition timing, and above all, your driving style. All of those can be adjusted.