My '76 scamp
had a hole in the same spot.
The best way to repair it is to replace the entire rear floor section.
The frame rails are very far apart in the rear and the center portion of the floor has no support for any sort of seams.
The floor is actually made from 5/8" 3ply interior grade sheathing on these older Scamps. It sucks moisture up like a sponge and is not rated for flooring use.
I ended up replacing mine with 5/8" exterior 5ply and it is a world of difference. The floor doesn't bow when people stand in the rear.
It really isn't a hard repair. You just have to take the seats out, remove the old floor, and cut a new piece to wedge in and fillet the edges with fiberglass.