nice BB dart ...crappy rear frame rails..broken mount

SOMEBODY........it doesn't matter who.........did a cosmetic job on the frame rails to sell the car (the spray foam is a dead giveaway as well as the self tapping screws through the trunk floor).
I don't care if it was a show car in 2002, it's a pile of crap now frame wise.
And it has been for some time.
You got screwed by the seller to make it mild!
He knew the condition of the rails, that's why he was selling it.
Period.
On a positive note, it can be fixed for not a lot if you can do it yourself, that is if it's only the rear frame rails.
Take a screwdriver or a chipping hammer to the frames and floors front and rear every inch and hit it solidly......... and i bet there's a lot more rot you can't see. Pay close attention to the torsion bar cross member as well and the top of the frame where the front frame rail mates to the upper control arms. These are usually the bad spots to look at first.
Let's not sugar coat it here, the engine isn't what he said it was, and the frame at least on the back is bad.
I'll bet the whole car is rotten if you really get into it.
Then take pictures and call him about it and if that doesn't work, tell him you are going to contact your lawyer.
Very unsafe car he sold you!