I have a seat belt kit from Seat Belts Plus that I need to install. Interesting ideas on the lower mounts. I used the plate from their installation kit on the shoulder mounts. Have a little more room than you did, because my truck is a 3 window cab. Thanks for posting the photos.

Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.