I've seen a couple of different attachment methods. (And there may be more.) On my 1974 (and others of similar vintage), there are screws that go into the frame. So, just through the fiberglass, through the floor core, and into the top of the frame. They dead end inside the frame.
On a late 70s Boler
, I saw that it had the same as described above, and then also some (factory) right angle tabs welded to the frame, and then some bolts going into those (through the fiberglass floor, through the floor core, through the tab, then, IIRC, a nut).
Amazingly, although I've seen a number of these where they are either missing or will just spin, I haven't heard of a shell blowing off on the road. Must be friction.
I think the late 70s method is a nice one, but on the other hand, when I was thinking of using bolts through the frame to re-new mine, several people pointed out that it is better, structurally, not to do that (and, their reasoning was sound). Given how many are loose or broken on so many Bolers, and how few seem to have given way, I decided I would be doing just fine by using the same method with new, slightly oversized fasteners of the same style (screws into the frame).
I have heard of people retrofitting with large U-shackles (like on trailer frames), but I don't think one has to do that (it's good though, of course).
If you are getting any core crushing where the screws go through, you could overdrill and fill with thickened epoxy to create an annulus. OTOH, if all is dry and fine, it's probably okay as is, since it's not an area where water will sit.
I started a thread on this about 9-10 months ago, if you wanted to search for it, although the upshot is summarized above.