Your 360 has flat-top pistons that are the same as 390 high comp. pistons. Swap in a 390 crank and rods (if your block and pistons are good) . A four-speed overdrive makes a good swap, especially if your truck is as low speeded as you say. They were factory optional in the early '70's. Its a Ford top loader style tranny. A bit of a jump from second to third, but a 390 has enough torque to overcome that. I have one in a '79 Mustang with a 351W and it works great.