How many bolts are turning?
is it just on the lower pillars?
If its the bottom bolt of the lower hinge pillar you may not be able to access it from the inner kick panel.
...pic... to weld a new nut you need clean rust free metal to weld to. That may be a challenge in that area
I would try grinding the heads off first and see if the rest of the bolt and nut falls in. The fine threads may be stripped. I like drilling out busted and stripped bolts with a left hand drill set I have.
Then you can determine how to repair.