On my 72 K5's top,someone who owned it previously must have had that happen,and they drilled two 1/8" holes right next to the bolt bracket,and put two pop rivets in them..
I'm guessing the "nut" buried inside the fiberglass is actually a tapped metal plate to spread out the load forces,and they were able to get the rivets in that plate,and keep it from spinning..
I took the roof off it several times,and used a lot of never-seize on all the bolts--only one other bolt in the rear corner did not want to come out,the inner nut or plate just spun around inside the top layers,so I decided to just cut off the head of the bolt--that left enough of a "stub" with a few threads,to allow me to screw on a coupler nut intended for threaded rods,and use a short bolt and a spacer made out of a piece of pipe so I could still bolt the top down tighter in that rear corner..