66 dart gt

Hello, and welcome.
I would not use fiberglass, It will provide no structural help, and will not hold up. It will eventually seperarate from the metal it is bonded too. Seen that a lot of times on trunk floors, and floor pans.
If you cannot find factory rear pans, fabbing some will not be a hard job. Then weld them in,
and restore the structural integrity of the floor pans.
I just put new front pans and patched the rears in a 66 dart Gt. It is not a hard job.
Keep us posted, specially with pics.
I used the roof of a 67 Barracuda, and made my own pans, and patches.

barracudadave67