i don't think the newer automatics are bad, but the older ones had quite a pit of power loss just to drive tranny. the manuals were the most eficent back in the day. if upgrades are not a problem you could run a automatic fine i'm sure. all the originals had 3 speed with granny type or sm420 so no real difference between that and saginaw 3 speed. the final drive ratio is 1:1 i believe unless you install a latter tranny overdrive unit.
i wouldn't say any six would beat a 8 on milage. the style of six for these trucks run 198, 230, 250, and 292. if it's just a empty 1/2 ton it will weigh around 4000lbs. the 230 would be kind of a dog but ok. i think the 250 would be fine. now if you are comparing it to a 283 not a big advantage on milage. seems like everyone wants to drop in a 350 and i would bet the 250 would beat it on milage. now i have not built a 350 and put it in my truck to compare so i can't tell you the exact #'s.
would be interested if anyone has seen a change in milage in swithching from 6 to 8 or back. if both were in comparable shape. i have owened some wore out v8's like a international 352 (i think) that got the milage of a school bus, so you couldn't compare apples to oranges.
