Just as info - the seat belts that were in my car when my Dad bought it new are 35 inches long each belt (front only).
Mfg by Irving Air Chute Co. Pretty sure all seat belts were dealer installed but since these came in the car it is possible this is what Ford sent to the dealers.
The reason I say that is because the car was bought in a very small town that I doubt very seriously would of had a store that carried aftermarket belts. Closest town would have been 2 hours away. Unfortunately the window sticker was gone when they got it so no definitive proof.
(funny but our Tbird found my Mom rather than she and my Dad going out to find it - the car came to the bank she was working at to be financed and the fellow was unable to obtain financing. The bank was going to call the dealer to come and pick the car up but my Mom called my Dad - they looked and the car and it went home with them rather than back to the dealer......