Sounds like your\'e into more than just the window motor swap!
They are completely differnt beasts! Personally, I'd take the 80, and make it into a nasty trail rig! Then you could restore the 78. Or depending on what goodies you got with the 78, fix up the 80 real quick and sell it for money to put into the 78... What engine does the 80 have? Does everything work? (4x4, tranny, electrical, etc...)Anyway, It sounds to me like if you take the tailgate apart on your 78 it'll probably be obvious what the problem is. Whichever you choose, enjoy your Bronco(s) and keep this board in your favorites folder. It's very usefull!