No SAFE B Pillar Mount For Shoulder Belt?

[Dan in Pasadena]

I've got a '55.2 with a big rear window and all I've read seems to say there's no safe way to mount a nut in the B pillar for shoulder belts except for small window trucks. I'll use lap belts only if that's the best I can do but would prefer shoulder belts.

Here's my idea, maybe it's overkill but I'd like to hear opinions.
Using a 4"-6" long piece of 1/2 square tube, drill a hole through one wall, insert correct sized nut and weld it in. Then, using a cutoff wheel cut a slit in the B pillar for this tube, insert it flush with the sheet metal and fully weld in, grind and fill til nothing is visible but the nut hole.

I know the B pillar isn't all that strong in a truly serious accident but otherwise do you think this would work? Is there another easier but safe way?

Pictures would be appreciated.