I've had to do it to my 84. I've never tried jacking on the fiberglass as it doesn't seem like a bright idea, but on the metal frame or suspension parts is always a safe bet.
I patched my hole and called it a day, been going for... 5 years now?... no problems. It's been a while since I did it, but I don't recall there being any structural fiberglass down there. Bet bet would be to jack it up, or better yet get it on a lift, and check for yourself. I'm pretty sure that everything important bolts to the frame itself and not the fiberglass floor.
If I'm wrong here, I'm sure that somebody with more experience will chime in.
