List your MPG on your transplant 5.7 or 6.1

The stock low cr 318 in my '73 Dart does 13-14mpg on average.
2bbl 318 / A518 / 3.91:1 / 8-3/4" rear.

The 318 runs on propane. On gasoline it would do prob. 15-16mpg.

If the 5.7Hemi I've planned to swap in the car doesn't get anything way above 20mpg on propane in this lightweight car I'd be kinda miffed to say the least.
On the other hand, the 5.7 would still mean a 200hp increase over the 318, so that would soften the blow a bit if mpg's aren't much higher.