The rear frame rails also have some soft spots by the tail pipe hanger, where the hanger cracked a section of the rail out, possibly due to the custom exhaust that is using only the rear hangers to support the entire weight of both pipes. Normally they should have a transverse muffler that is fully supported by two brackets above the rear differential, sharing the load.
And the lower area in the normal Firebird frame rail bad spot:
Here's a quality repair - use spray foam in the rocker hole. It seems to be isolated to the passenger side inner/outer fender well seam: (By the way, look at the assembly line sticker still on the original brake drum!)
Once all the foam was picked out:
