79 I/O Rear seat question?

[breastman569]

So i have two pedistal jump seats that i want to mount on either side of the engine hactch, what my question is, is what is under the floor? Is it just a think sheet of of fiberglass or is it something like 3/4 ply with fiber over top? I have them mounted to plywood wood now so that they are removable but it seems that they never come out so i would like to go with a cleaner look. thanks!!!