Is a Holley 750 DP too much carb?

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.