I had a '73 up style bed on my '71 GMC--it was free and un-rusted,so I decided to use it,even though it did make the purist in me throw up in the back of my mouth a bit..it bolted on with 6 out of the 8 bolts and the wiring harness plugged right in too..so it was fairly painless to install...it did look a bit goofy,but the bed was almost a perfect match color wise,so I lived with it..
I have seen some newer '88 up style stepside beds on older square bodies,they dont look that bad,but the fleetside newer beds only belong on the trucks they came on in my opinion..
There is a 1970's square body around my area with a Ford stepside bed from an 80's vintage Ford that doesn't look that fugly,but having a "Ford" tailgate on a GM is just wrong !..