The cage nut holder on my right mid mount body mount just above the trailing arm has broken off inside the frame rail, and I can not remove the body mount or retighten the bolt in that location! It was near the end of the bolt and is screwed out a couple of inches! 

I was doing a body off lift to install new mounts, bolts and to install some fuel lines.  Is there a way to access that frame mount nut inside the frame????? 

All the other body mounts bolts have come out but the right mid mount nut has broke loose near the end of the bolt.  So I can not remove it or retighten it since the nut turns with the bolt and I have no access to the nut to hold it!

I can cut the bolt off, but then the nut and bolt stud would fall down inside the frame and I would not have a frame nut to remount the bolt when replacing the body to the frame! 

suggestions?  

Assume I may have to cut the bolt off, raise the body, then cut out an access hole, remove the old nut and re - weld in a new nut into the frame!   It appears I could cut an access hole at the end of the frame to give access or cut one where the bolt goes thru the frame!  

Is there any other way to do this?

I’m confused by your situation. I revisited the old pics of my frame when I had the body off to refresh my memory - the cage for the nut is on TOP of the frame, not in it:

Is that what yours looks like?

If so, you can just cut off the old cage/nut/bolt and weld on a new cage and nut.  If anything accidentally drops inside the frame, you could get one of those magnets on the end of a flex rod and fish it out with that. There are access holes in various places on the frame. 

If not… post some pictures. Let us see what you have. 

Here's a good resource: https://www.corvettemagazine.com/tech-articles/c3/frame-and-chassis/?utm_campaign=284184_1963-1982 Corvette Body Mount Replacement&utm_medium=email&utm_source=Zip Products Inc.&country=US&dm_i=6MYT,63A0,1EALG6,UCF9,1

Great information!  Think that solved my issue, since I have not lifted the body off I had assumed the frame nuts were inside the frame rails on the back side inside the frame but it appears the #2 and #3 frame nuts are on top of the frame!  That solves my problem!

thanks

the zip articles help a lot!

That solves my problem!  Makes sense!  Your picture shows me it is on the outside of the frame!  I just have not lifted it up to be able to see that.  Thanks for the photo !