I agree that the 81 was a unique year and the center stands were not designed properly. I have a Reynolds ride off on my 81 R100RT. You have to have the proper model number if you want to get a Reynolds stand.
Yes the bench seat will fit on your bike BUT you can not use the seat fairing or the tool tray that is bolted on top of the rear fender.
A friend of mine actually put the bench seat on his 82 RT and cut and fitted the back part of the seat fairing to make it look better. Being the crafty type he also built a set of spoked wheels for the RT along with many other custom touches.