Check my picture album in signature for 51 GMC. Believe I have detailed pictures of that process and they fit and work nicely.
Cutting holes for the straps in the seat frame is key to good belt routing in the AD trucks.

I’d use shoulder belts if/when I do it again, lower locations stay same.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-