Slant 6 most mpg's

OP, I drove a '76 Dart Lite 4 speed for about a quarter million miles. Stock, it got around 28 mpg at 65 mph in the mountains on the interstate. In the flat midwest, 29 mpg was normal for 65 mph. With the approx 400 lbs that they claimed to have taken off the car, it was plenty torquey. It would mildly ping on hills when lugged down in too high a gear but it never hurt anything.

I'd leave it stock. The only 'mod' made on my Lite was a vacuum gauge under the dash.... it sure teaches you fast how to keep your foot out of it.