Well, I'm no expert, but It's clear that the B-Pillar fastening points on modern car carcass has been specifically engineered to hold in the event of an accident. The 68 was not.
Therefore, you either need to properly reinforce the upper B-Pillar point on a first gen, which, I suspect is a huge undertaking due to the absence of an actual B- Pillar. Alternatively, you can just weld a plate in that area, and hope you are never in an accident.
I suspect that it would be safer to use just the original lap belt, than to use a modern 3-point belt system in an improperly engineered retro application.
