Hi there ,
Did this kind of repair the last couple of months and i can tell you right now its always worse than it looks !
Looking at your pictures it looks like the panels that fixes the reactionarms to the body are eaten by rust , and also your sills don't look so good .
If this is the case the beams that go to the back and over your wheel arches are likely to be gone as well . Probably not a California car ?
I used a sort of digital pinhole camera , very affordable now , and this gives you a scary inside look
Replacing these requires a bit more than average skills , but the good news is that replacement panels are available and with the right
equipment ( MiG welder , Carlift ) its doable .
Good luck !