Mikael,
You may (hopefully) get lucky and find the previous owner didn't have the correct bolts and just used a smaller nut & bolt leaving the old threads intact. It is pretty hard to strip out those fine UNF threads.
I believe in most Country's it is a DOT requirement to have the bolt thread into the chassis/body and that removable nuts are prohibited.
The 'captive nut' on my '74 is actually a piece of threaded tube welded into the box section, it does not go all the way through to the engine compartment. I would think it possible to cut this out and replace from the cabin if necessary. The tube is very thick wall and IMO you could probably retap it a slightly larger size. If so use an equally fine thread.
Are you using conventional seat belts or harnesses?
Good luck,
Julian
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.