Has anybody had to deal with this before? How can it be fixed/replaced? I was going to cut the bolts out and remove the whole bolt/welded nut assembly and replacing with new hardware. Hopefully I will have enough metal to weld in a new nut. Any suggestions?

which ones?

You should be able to access the rears under the front seats. I was just under the front seat last night and saw the top of the mounting bolt. The middle ones have an access hole opening towards the front of the car. The radiator mounts should be a snap!

Hope this helps...?

I have new floor pans ready to go in. I already have the front clip off and I can see the welded nuts for the middle mounts. I have already removed the front ones when I installed my new rad support. My problem is, when I try to remove the middle bolts, the entire nut assembly on top is turning. It appears to be inside of a steel 'case'.

I had the same thing. I cut the bolt head off, used a pry bar to remove the cage, then took the rest of the bolt/nut out and used regular bolts/washers/nuts - grade 8. The cage is tacked and comes off pretty easy.

Thanks Rick. I thought that was the way to go, but I did not want to just jump into it without asking if it would hurt anything.
Also, do you have any info on removing a bottom fender bolt that is turning because of a busted welded nut? Is there a way to get to the nut?

Mine's still inside the cavity! Sometime when the bodywork gets done, I'll have a small square cut out, remove the loose nut, have another wlded to the patch and have it welded together. Others have done this. Right now I run around without that bolt in.