Upper seat belt bolt/nut question

The upper seat belt bolts in my recently purchased Pantera do not tighten properly, and are difficult to get out and in, because there's a nut in the engine room that mostly turns with the bolt. I need really long arms to hold the nut in place and push and turn the bolt at the same time...
The previous owner probably didn't care, because he had a roll bar and a plate where the middle part could be removed, so he only had to tighten them once, but I have a new full plate, and have to have the seat belt bolts on and off every time I want to time the ignition etc.

First question is if the bolts are correct. If they are, then the original thread is gone. If they're not, does anybody know what length, diameter and thread they are?

Second, if the thread is gone, I had this idea. Push a bolt through from the back, drill a small hole in the "pipe" sticking out and weld the bolt. Then I'd only need a nut. I could use a normal nylon lock nut, but would like something that would give a positive lock, like e.g. the system on headder bolts. Any ideas? (I have 4-way belts on a seat belt bar). Safety is important...


Merry Christmas