What's to much hassle mean to you?
IIRC the only V6 that came in the 85 models was the 2.8L, so an 83m 84, or 85 would be the least hassle. If you didn't mind swapping in quite a bit of wiring any year 2.9 would fit. That also holds true for any 3.0 or 4.0 used in Rangers, but they would also require transmission swap. I believe the same goes for most of the 4 bangers.
For about the same amount of work as a 3.0 or 4.0, you could put in a 302 or 351 (neither of which were ever stock in a Ranger). Doing so would require engine, transmission, driveshaft, rear end (possibly), cooling system and wiring mods how much depends on whether you go EFI or carbed.