It depends on what donor car you have under your TD. I suspect it’s a VW because your signature says FiberFab MIGI.
Depending on what year your floorpan is, there should be seat belt anchors that were used for the back seat. If not, I suggest welding a plate to the areas where the seat belt will achor and then bolt the seat belts to the center of that plate.
As far as mounting a shoulder harness, unless you have a roll bar, you’re pretty much not going to have a spot to mount one. If not mounted properly, shoulder harnesses cause more damage than they prevent. Search the web for proper mounting of seat belts and shoulder harnesses.
A key thing to remember, especially with VW based TD’s, is that you are basically driving a 4 wheeled motorcycle with a faring shaped like an MGTD. The chopper gun fiberglass shatters like balsa wood when hit.
The Ford and GM based (front engine, RWD) TD replicas have full tube steel frames. That affords the mounting of steel roll bars, 5 point harnesses, etc. These cars, although not Volvo’s, are much safer than the VW based TD replicas when built correctly.
I’m sure I just started a controversy! At least some more forum action may result…who knows