1970 blazer fiberglass top bolt size.

[Eotku]

I bought a 1970 blazer and it was told that it originally had a camper on it. They sold the camper and put a double wall top on it. Right now it is only attached at the windshield and I now want to attach it at the rest of the points. I put a small screwdriver in one of the holes and was able to line it up but I tried putting in a 5/16 bolt but it didn't take. So what are the size of the bolts needed for under bolting the sides? I also need to drill holes for the back two mounting points at the tailgate and would like to know what was stock since right now I have no holes at all. I am thinking of putting in nut inserts. I would appreciate any info and help you can give me!