Since you have a flatbed you could use just about any rearend you want. Any truck rearend other then a 47.5-55.5 would be a tad wider, but then w/a flatbed it really shouldn't matter if the rearend is a couple of inches wider as long as it's still under the bed.
I'd look at any truck w/an automatic trans, then just pick a ratio and go looking, personally I'd get a rearend w/gears in the low/mid 3's. It will still give you good pick up yet your motor won't be wanting to jump out of the engine bay because of high RPM's &&&& you'd be able to commit a crime
by breaking the speed limits w/o a problem.
I did a swap years ago for a friend who had a 3/4ton, she wanted disc brakes & late model rearend. So I swapped out the 3/4ton front axle for a 1/2ton and used a reareand out of a mid 80 fullsize GM car. W/the stock motor & 4spd trans w/the rearend I put in the truck easily did 65mph w/out problems....Joe
