I had a 305E (pick-up engine) in a '62 GMC 4000 16 Ft Flatbed and have to say, fully loaded it pulled pretty good. It only had a single 4-Speed and no 2-Speed Rear to boot. I say fully loaded, but it was actually overweight, carrying 2 1/2 Cords of green firewood. Now it wouldn't win no race, lol, but it pulled surprisingly well. My '62 6500 with a 478 and 5 and 2 would also win no race, although it had power to spare fully loaded at 32,000.
If the GMC B above was cheap enough and would not break the bank to get it home, I would probably buy it with the intention to use it for parts. If it proves to be otherwise, you could always restore it.
