I had my mechanic change the body mounts because the bushings looked dry and cracked. He twisted out 3 of them. What can we do to fix this short of taking the body off the frame? the car is a convertible.

My opinion you will have to use a saw and cut the floor where the bolt is. If you use a hole saw it will be a clean cut then get a piece of old steel and use the larger hole saw and cut a patch to cover it up. Pop rivits and under body coating should do the trick. Remember to primer and paint everthing in there so there is no chance of it rusting. Or the old methoud of drilling out the bolt and retaping but it would be tough because the frame is so deep.

a hole saw is one technic, another is the Trap door. , Thats where you use a Wizzer and cut a three sided flap( trap door) and pray it UP to gain access to the spun weld nut.

Then spray with weld through primer. You then push flap down with your handle of your body hammer and weld closed..