unibody repair on a 74 javelin?

[nelsonstrange]

I just bought a 1974 plum Javeline the body shows no external rust.
Althought there are bubbles in a few places under the paint.
I am going to put it on a lift this week. I would like to know exactly what to look for in determining whether or not the unibody it beyond repair.
The guy I bought it from said it is sound and only the floor boards have been fiber glassed. From clawing under it it looks pretty rusted.