Technical Three Point Seat belts

You could make a wraparound metal bracket that goes 360 around the rollbar tubing, with tabs extending that would sandwich the top 3 point piece. No welding and much stronger than a muffler clamp. I believe this is also better than trying to add a mount to the B pillar.

 

That door post on your 50s car is most likely attached to both the roof support, and the rear quarter window frame and a belt line brace that runs around under the back window. It is also the same support that the door latch is incorporated into, though that lower section may only be a U channel rather then a complete enclosed area.

Adding the proper belt attaching hardware to that post (better if the hardware was down inside of the post rather then just attached to the inside panel) will be about as safe as any other area to mount a seat belt to in that car, an accident would have to pull that entire area past center before the belt would become loose, at which point your still way better off then you were without the belt.