72 slant six upgrade

No, not really any correlation between carb barrel numbers and mileage. The Dart Lite's and Plymouth Feather Duster's got the high 20's on the interstate at 70 mph with a 1 BBL carb (with the manual trans); I know since I drove one for most of a quarter million miles. Better gas mileage for a given throttle setting and speed is a matter of better combustion efficiency, lower engine losses, and/or fuel mixture. This can be due to:
- Better fuel-air mixing due to a lot of factors
- Better fuel-air distribution due to a variety of factors
- Leaner fuel-air mixture at the most common driving RPM's
- Lower engine speed due to different gear or trannie (the Lites and Fethers Dusters had a 4 speed overdrive manual trans); that lowers the engine RPM's and that lowers the engine pumping losses
- Larger exhaust improves exhausting the spent gases and makes the combustion cleaner and more efficient; it also helps cylinder filling which helps power.

I would not expect better mileage with a larger carb but it all depends on the carb and a bunch of other things. Your 1 BBL sucking the gas means either the 1 BBL has a problem (that can very likley be fixed) or you have a heavy foot.