600 vs 750 Holley

Thermoquads used on stock 360's were way bigger then 600 I would tend to think your under-carbed too. Your using 70 jets now in the primaries and 3310's would normally have 76's. It's only 75 or so more primary cfm. I would think if the secondaries were well tuned and you drive the same you would see little difference in MPG in everyday driving. I ran a 3310 on my 70 roadrunner back in the 70's and could get 16.7 MPG on the highway with 383 /727 and 3.23 gearing.