I was not aware the GM used inserts that early.
My guess is if you cut the bolt and drive the insert in it may leave enough material to use an aftermarket nut-sert. I did this on my 71 Lemans even though the holes were not stripped but getting "soft". I did make a PTFB type backing plate but the uneven surface of the inside of the rail made it useless - I then used a nut-sert.
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