MPG Motor Magnum vs. Hemi

AJ, help is much appreciated.
I've only driven with this 360 engine (and cam) for about 3 weeks. And had 3 tank fill ups.
But till now the mileage has been the same with the bone stockish 318 ('88 block w/ stock rollercam) and early '80s "302"-heads.

I have no issues with swapping cams, but if I do I want to step up to a (solid) roller.

Propane make less power in the same built engine, but if the build is optimized for propane it can come very close or sometimes surpass gasoline power.

When I got my Dart it had the 318 in it with a 904 transmission and a 7-1/4" 3.23 rear axle.
It could (once) manage 17mpg on a long highway trip, keeping it below 65mph.
(Fact was the engine started running hot if went faster so had to drive just over 60 all the time).
Found out later the engine was running very lean too.