I am doing my first frame off resto on a 72 chevelle. In the process of removing the Frame from the Body I had six Body Bolts spin on me, all in the rear end. What is the best way to remove the bolts and what way would be recommended to reinstall them?
Thanks.

They use what a called "captured nuts" with the nut fastened to the inside. The only way I can see it to cut open the cavity they are located in and fix it. How you do that is up to you, you can butcher it or find a professional welder to do it but I see no other way.

Following what elcamino just stated, they are also called " cage nuts ". The cage holds what should be a square nut. The cages may be rusted, hence, weak. Once you try to remove the bolt, instead of the cage acting like a wrench to keep the nut in place, it bends, allowing the nut and bolt you're working on, to spin.

If you are able to jack up the car and let the frame drop down while supporting the body, you may be able to get a hacksaw in the gap and saw the bolts in half. You will, at some point, have to cut into the floor of the body to expose the cage nuts to either replace or repair them.

Another way would be to support the car by the frame and cut the bolt heads off with an angle grinder fitted with a cut-off wheel. Then drop the frame out while supporting the body. On my '66, in some cases, I cut openings in the floor above the cage nuts then jammed a screwdriver (or two) to wedge the nuts and then broke them free. In other cases I cut the floor, air-chiseled the cage off, and wrenched the nut. Mr Orgill is right, the floors will likely have to be cut at some point.

On my El Camino I had to cut an access hole in the body support channel inside of the body mount. This would be from underneath. Then working through the hole I beat the cage off and stuck a 1" wrench in there on the square nut.

Thanks, for all the info. Looks like I will be cutting some holes. I am planning on installing a new one peice complete trunk floor pan and a one peice floor pan any way.
Thanks again

I noticed that I failed to point out that my suggestion to cut the heads off only applies to those positions where the head is accessible - the four rearmost and maybe the ones directly above the rearend...d'oh!

on my 71 elky i had same problem, i cut them off w/ sawzall and then used a high speed cut off wheel but first i made a small square on body around area to be cut out then cut along the lines. there is a double wall of floor so you have to cut them both. once out get yourself some new nuts clean up the metal pcs you cut out and either tach weld the nuts in place or make a new cage for the nuts to stay in and tach them. once done you'll be able to weld each square back in it's original spot and on the outer one you will be able to finish it off clean since the space your welding was made by cut-off wheel. use some good weld paint to coat the back sides of surfaces before you weld. the first piece when done can be sprayed w/ some rust encap paint.
hope this helps, don't get me wrong it is still a pain in the butt job that just has to be done.

On my 66 the cage nuts are installed on the body brace, My plan is to drill out the spot welds and replace the floor brace which at the same time will replace the cage nuts, used a torch very carefully to cut off the heads, worked good

Thanks, I did part of what you had suggested. I cut a square hole over the cage nut and chiseled the cage away so that I could get a visegrip to clamp on the nut. Then I could just remove the bolt instead of cutting it off. This I did on the back four so far, I still have the two to go, these are the two under the rear seat just before the frame goes up over the axle. Also, thanks for the info on welding new cages and sealing the square hole with the cut out piece.
Please bare with me, as this is just the begining and I have a long way to go restoring this car. I will probably have alot more questions as time goes on.
Thanks everyone!