Smallblock efficiency or How to have your cake and eat it too

Good topic.

I run a 11.3:1cr 360ci every day in my '73 Dart. 0.030" overbore. Propane fueled.
318 '302' heads, CompCams XE256h, converted to solid, KB-pistons w/balanced crank assembly.
A518-OD (no LU), 3.55:1 gear ratio in a 8-3/4" rear axle.
MegaSquirt ECU controlling ignition only for now. Spent months tuning to come up with a curve that works nicely for the engine.

The car doesn't get even near the mileage I had hoped for... 11-12mpg, mixed highway+street.
Highway only driving barely gets 13mpg.
I'm mostly blaming the A518 currently, and its most likely unmatched mid '60s stock stall convertor currently in the mix.