It may be a difference in how the truck is designed.
Trucks with seat belts from the factory may have been designed to double them up, so then it would be OK.

The do it your self drill your own holes kits may not have the strength to double them up, or the suppliers just want to be on the safe side and avoid law suits?

•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-