The cam and gears are the main culprit in your gas consumption. IMO, I think you need to get into the tune of the engine. Carb & distributor.
What ignition are you running?
A move to 3.55's would only help a little bit. And yes the 750 would still be OK with those gears. A move to a 650 would be OK, mileage wise, but again, you'll only see a little bit. You will miss some top end power with a 650.
I sure do appreciate your advice sir. The ignition is a Mopar Performance Electronic ignition kit. 6 AL Ignition Control Box, rev limiter chip at 6000 rpm. Summit coil, timing set 10 degrees advanced 38 degrees.