Man I love all of these old trucks! I was hoping to get in on the old truck game over the weekend but the traveling logistics didn't work out to get to the truck.
I miss my 81 D150, it was a \6 with the OD 4 speed. Averaged 20+ highway.
You can get a slant six going pretty good too . My neighbor just picked up an 87 short bed and he wants me to put headers , intake & carb on it . I'm even thinking of putting a bump stick in it as well . I think it'll be pretty good .