I did the bumper slide on my '75 today. When I was bolting it back together, the weld holding the nut inside the frame broke loose. Now it just spins and won't come off. I can't get to it inside the frame to hold it either. The crossmember is on the inside of the frame so I can't drill a hole for access. Does anyone have any suggestions?
The back bolt is the problem. Ignore all the cosmoline. I had it rustproofed when I bought it in 1991 and it doesn't come off easily.
You could drill all the way through and use a longer bolt with nut. Or to do it right like this guy did. http://mmb.maverick.to/threads/big-to-small-bumper-conversion.44352/
A nut was welded to the back side of the washer, the washer welded to the frame. I had to look several times to figure it out. I like how he accomplished his goal.
If you have a mig/stick welder its not a difficult fix. You will have to cut the head off the bolt though. Cut the head off the bolt...Get the bracket out of the way. You now have a bolt that is a stud. Put vise grips on it so it does not fall into the frame rail. Drill a 3/16" hole right next to the bolt hole. When done you should be able to see the captured nut flopping around on your stud that was a bolt. Drill another hole opposite the first one... Now you can pull the captured nut against the rail and rosette/plug weld it to the frame in its original position useing the holes you drilled. It will now stay in place so you can remove the stud that was a bolt and replace it...And bobs your uncle. If you have a welder...