When I was a lot younger (20 yrs ago+) we had a 66 long nose stroker with static belts in the front, and I'm pretty sure they were not attached to the b-pillar but to the metalwork between the rear window and the trim panel. This seems to fit in with the holes in the photos of your car (which looks very nice
). I think our car had the V4-type metal hoop between the seats for the belts to clip on to.
What I have done to fit inertia reel belts into V4s without the proper rear trim panels is rather than mount the reel on the sill behind the trim panel, to simply mount it at the base of the B-pillar, where the static belt would bolt to the floor & use the normal b-pillar pivot point for the the same purpose.
This may be viable with a stroker, you would need a captive nut welding under the floor (and possibly a strengthening plate) and the same in the b-pillar, this will be more tricky.
Mind you, I actually prefer static belts, but I'm a bit weird!
You might do better asking the same question on VSaab. Good luck...