Well, I’m ashamed to tell you this and now I can expect some chastisement! When I bought my Fiberfab VW, I knew it had a ton of issues and and replacing the pans was way down the list! (It WAS cheap!)  So I took a shortcut that is in the temporary category. The pans had some holes in the seat area, but there was still sufficient good metal. Get the battery out from underneath the seat, if yours is there!  I took the seats and carpeting out and cut down new floor pans and installed them on top of the fiberglass, effectively sandwiching the glass between the old and new pans from the petal assembly back to the tranny hump. I would say that the fiberglass body shell in the floor is close to 1/4” and I would not consider taking that structure apart.  They were fastened together with a lot of pop-rivets. Then I got some steel flat stock (two pieces) and positioned from from side to side full width and anchored both seats and belts to them.

The only way to do this right is to take the body off, which I think is entirely possible and not the end of the world. This winter, I’m hoping to disassemble most of the body and get new welting and and take the fenders in for paint.  That will be my time to do the chassis correctly.
I’ve installed a new engine, rewired everything, new top, new interior and new dashboard. And solved the overheating issue! The body is my last thing on my list.  Good luck,  Gregg