Slant 6 MPG Upgrades?

How much do you want to keep running a slant-6? The 225 is inherently inefficient by design with the super-long stroke and heavy rotating assembly. If I was to ever intentionally own a slant it would be the 170 variant. I think that would be the best bang-for-buck move in search for MPG and keeping the car 6-cylinder. Could just swap over the upgrade parts you already have. The slant wasn't intended to go over 170 cubes, when Chrysler management asked to make a bigger version the engineers wanted to increase the bore but the Valiant designers were fixated on the length dimension of the engine bay so the only option was to increase the stroke.

IMO you'd be way ahead swapping in a stock 5.2L Magnum V8 with the factory multi-port EFI. Only potential issues would be needing a different intake manifold to clear the hood. Mounting options aren't tough to come by, either conversion mounts with current K-member or find a V8 one. You could use factory 273/318 A-body exhaust manifolds which nobody wants and are dirt cheap and could easily find a V8 904 transmission to bolt in place of your current one. Even with the current gearing you would easily surpass 20 MPG average in such a small and light car and you'd easily double the HP as a bonus. I had a 1988 Chrysler Fifth Avenue that averaged 20 MPG and that is a much heavier car with a roller-cam 318 and 904 trans with lower 1st and 2nd gear ratios. My average MPG didn't even change after I swapped to 3.07 gearing (they never came with overdrive), 4-bbl carb and intake and 2 1/4" dual exhaust off the factory manifolds as well as ditching the spark-control computer for conventional distributor and used MSD box I got for free from a friend.