Slant 6 most mpg's
My DD is a 73 Duster with a 225/904/2.91 gets around 24mpg
.080 decked (78)peanut plug head
Erson RV10M cam
SuperSix 2bbl setup
MSD with recurved distributor
2 1/4 exhaust
TransGo Shiftkit
Clutch-fan setup
future upgrades will be a 78 hydro lockup 904 and possibly FiTech efi
shooting for +28mpg
Good luck in your mpg quest.