We acquired some non retractable 3 point seat belts from SteveJ after his visit. I knew there was a mounting spot behind the vinyl in the B post, you could feel it. Went to install them today and cut through the vinyl. Surprise! The hole is there but no nut to bolt into. So now what? I need some ideas for getting a nut inside the B post up high and fixed without butchering anything...

Charles, I think that might be too low. Look at the photos in this thread: http://community.ratsun.net/topic/7800-nice-aftermarket-retractable-seat-belts-for-510s/

I will look up that high but if I mount them that high they would take your head off in a wreck. The belts in that picture have a long stick extension that moves the shoulder belt down beside your head...

I noticed that, too. The link provided in that thread redirects to https://www.seatbeltplanet.com/

Maybe the 72 510 had retractable belts and had a similar mount.