Best possible mileage?

My 93 318 dakota club cab got 22 over the road consistently after I converted it to a 5 speed stick.
Any 3.9 powered dakota I have ever had seemed to top out at 17, occasionally 18 but not very often. I'll take the v8 in that case.
Slant wise, my 79 D100 with the 3 on the tree manual steering and brakes and 3.55s was good for 22 over the road.
I have an 85 D150 right now, haven't driven it enough with the original engine in it to judge mileage, just got all the bugs out of it, and winter came. It's solid and want it to stay that way.
I have a 74 slant 6 that I'm building to play with in it, cam is at Oregon cams as we speak, block in machine shop, just finished porting the head as far as I dare (the band sawed one all over the forums has me scared, I may have already wrecked 2 heads, have 1 more I can experiment with) have an NOS BBD for it, hogged out exhaust manifold, have de-lean burned it already, factory 4 pin ECU.
Going for best I can get from it considering the 727 and 3.21s aren't going anywhere anytime soon.