Hi Tor,
I received your PM, but could not reply to you via PM. Forum would not allow it for some reason.
AMK makes 2 kits for your seat belts for your 70 shelby convertible - front and rear. See below. I suggest you verify the place nut threads are still in good condition and proper size before ordering your new kits. Sometimes the threads are damaged on some cars.
You can get them at amkproducts.com or other mustang vendors. Good luck and it sounds like a very nice car.
Part num
F-3561 SEAT BELTS (Fr) TORX BOLTS, RUBBER WA'S, SPACERS 12 16
F-3566 SEAT BELTS (Rr) 2-29-68 > TORX BOLTS, SPACERS 8 14