David,
These photographs from the quickfit website give you an idea how they approach the issue of putting seat belts into Bristol cars. As you can see they use the rail under the side windows for a mount whereas on my 411/3 the inertia reel was on the innner wheel arch and the belt came out in the cubby box area, so quite a long run over the back of the seat, there was no mount in the area immediately behind the door like these photos show.
On the 603 the inertia reel has been placed in the rear seat well and there is a mount through the trim below the rear side windows, so a more traditional three point arrangement, for that Bristol had welded a mounting point below the B post, I am happy if you want to take some photos of the arrangements on that car. Quikfit might be able to sell you all the parts needed but its a matter of taste if you could live with their arrangement for the mount through the side rail wood trim.
Geoff.
https://www.quickfitsbs.com/gallery_bristol.php