Trying to remove front bracket bolts on my 70 Cuda and the nut on the inside of the frame rail is also turning so I can't take it off. Only acces to nut is through a small hole in the frame rail but I can barely get my finger through the hole much less a wrench to stop it from turning. Any suggestions that don't include cutting the frame rail or enlarging the existing frame rail hole?
Thanks!

extension and a socket?

sometimes you can get the biggest flat screwdriver that will fit through the hole and have a helper try to wedge the blade against the nut and something inside the "box" to keep the nut from turning. i have had to do this many times over the years. you may need something other than a screwdriver. i have used chisels,flat stock, wrenches[jamming the box or open side against the interior nut, not as you would normally use the wrench], really anything that would fit through the hole and jam up the interior nut from spinning. just throwing out things that have worked for me. the worst thing to happen is you would have to cut open the rail then repair as necessary.

Stick weld. The electrode rod should fit right up through the hole. You can weld the nut back to framerail at least 1/2 way around, which is probably more than it was originally.

Common problem, I think. I actually drilled a tiny hole on the inside of the frame box, opposite the nut, for a WD-40 straw and hosed them down for several days before removing the bolts. Sounds crazy, but I never had a nut spin on me. I think Ehrenberg recommended this years ago.

thanks! All good suggestions, I'll try them all.

And just think how great it would work with a good penetrating lube, instead of WD40!

But any lube is better than nothing,

Correct, E-Booger even showed a drawing about soaking K frame bolts to prevent this, I still have that issue somewhere. As far as the OP I had the same problem with a bumper bracket bolt , I used a Dremmel and a small cut-off wheel to slightly enlarge the hole .