Captive Nut Rear Cab Support

[farmallboy15]

Well I'm starting to put my 70 GMC back together, I have all new cab supports and bolts. All bolts were snugged up before I put the seat and gas tank back in.

Tonight I tried to torque them down to spec when the drivers side captive nut stripped I believe (can't see it, gas tank is back in). The bolt will start to snug then pop and be loose again.

Does anyone have any other ideas besides a stripped thread (I wont really know until the tank is back out) and an easy way to fix this? I'm assuming I'll have to take off the cage (spot welded??) and replace the nut. Also where to get the nut, I'm thinking McMaster Carr.

On another note the fenders don't line up quite right after putting in new core support mounts. The mounts seem to be too tall 3/4" vs. probably 1/2". Just cut a little off them? This is what spurred me to go over the cab support torques...

Thanks ahead of time! You guys have been very helpful. Pictures of the truck will come very soon!