Best possible mileage?

Yup. Typical 360. Gas sucking pigs. I've had better overall service out of 318s vs 360s. And I've had 318s that were better on gas in the otherwise same vehicle, than either a slant 6 or a 360.
I've had this experience when compared to either a slant 6, or especially the 3.9.
I've also had a couple of gas sucking 318s, the worst of which was my 88 D-100 short bed, usually good for about 8 mpg, at 70 with a good tailwind I got a best of 12. And that was with ALOT of good money after bad in pursuit of making it improve from there. I'd have been money (and results) ahead but swapping intakes, putting a carb onto it and either a 4 pin ECU ignition or HEI than all the money I threw at the damn TBI that was on that truck. I've had 4wd. Ramchargers and pickups that were better than that turd.
I don't drive trucks for MPG but that's what I meant "Best it can be for what it is". And when I can get 19 out of a 3/4 ton B van with another 318 something is wrong when I got 8, out of a short bed 2wd pickup. With injection mind you.
I'm hoping for 18-20 out of the slant 6 I am building for my D150. We will see. That is alot of weight for such a small engine in a pick-up.