Zleviticus -
Keep the 216 but swap out the rear to higher gears. You'll get the best of both worlds...original engine that'll do ok (not great, but ok) on the highway.
I used to have a '51 Chevy Fleetline that would top out at 55mph (engine was screaming). Swapped out the 4:11 rear for a Powerglide rear w/3:55 gears, and it would run happily all day at 65mph. Never even noticed a difference in performance, either.
If you can use the center section of a Chevy car rear in the truck housing, it's an EASY swap. Have all the parts in hand and you can do it in half a day.
Chris
Chris