Mine doesn't have the corner windows, but my experience may help. Got a set of belts (Dodge truck, actually. They fit better than Chev behind the door pillar.) Made a mount plate like the original. For the top corner, welded BIG fender washers to the mounting nuts (Grade 8 to match the original collared bolts). Drilled 4 holes: 1 for the bolt (9/16 if I remember), and three 1/4 spaced around the bolt hole, close enough to be covered by said WASHER. Taped the nut/washer to the open end of a combo wrench (tied a string to the box end in case I dropped it) and lowered the nut/washer down behind the inner skin near the pillar. Used a short bolt to hold everything from the inside and plug welded the WASHER in place. To keep the seat belt bolt from backing out, use thread locker and/or SLIGHTLY deform the nut prior to welding to the washer (thread a bolt in about 1/2 way, put the side of the nut on an anvil and strike the side of the open end with a hammer to oval the bolt exit side. SLIGHTLY.)
If my method is unsafe, I'm sure someone will chime in. I'll appreciate it as much as you. The belts are firmly anchored and easy to use.
Current fleet (subject to change w/o notice) \'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run. My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787. Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.
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