Sheet metal help.
 

I took the rear seats out of the 80 firebird. Removed what was left of the seatbelt. Found a pretty good size home. I bought a new floor pan. Cut out the driver side rear seat pan. I can’t help but think there should be something else under the pan to connect to wheel wheel. But I don’t know what it is called. Any help?
https://cimg5.ibsrv.net/gimg/www.pon...3185bdfe7.jpeg
https://cimg9.ibsrv.net/gimg/www.pon...1eaca10b6.jpeg
https://cimg8.ibsrv.net/gimg/www.pon...bba101a8c.jpeg

Would that be the inner wheel well tub? If so, can I put the tub on with out remove body from frame?

star bolts to remove seatbelts were a pain. Going to put hardened hex head in instead. I will need to weld the nut on to something, not sure what.

any help is always greatly appreciated!

If you're going to weld the nut on, I'd fabricate a plate several inches square, of 1/8 plate to spread the load. You don't want the seatbelt mount to tear out when you need it most.