I'm trying to put together a set of bolts for my seat belts. It's for a 69 Coronet with bucket seats. I have bolts in 3 lengths and I'm not sure where on the car each goes. I have an idea, but would like the wisdom of Moparts to prevail.
Thank you in advance! The car also has shoulder belts.

my guess............
starting from the left..........shoulder belt in the roof, lap belt at the rocker/floor, and the longest at the inner lap/shoulder belt drive shaft tunnel/floor mount.

Seat belt bolts tighten against the shoulder, or the ends of the threads, pre metric are usually 7/16 fine thread, grade 8 bolts do work (field tested in dirt track race cars). The length of the shoulder depends on how many belts use the same bolt, or how much spacing is required for the belt(s) and whatever spacer may be required to clear things that may damage the belt webbing (think inside plastic trim, or other belts). If more then one belt uses the same bolt, the belts both have to be able to move freely, independent of each other, the webbing of the belts can not rub against each other.

Thanks for the replies. I'm thinking the longest bolts are used where there is carpeting, therefore the front buckets floor mounts.. The shortest bolts may be in the rear, where there is no carpet and a single seatbelt attaches. The two midsize bolts in the rear go at the inner two places that have two seatbelts attached. I have no idea what length goes for the shoulder anchors along the roof.

This fastener spreadsheet is from a 1969 Barracuda 2-door hardtop with carpet and bucket seats but might be the same as 1969 B body 2-door hardtop with carpet and bucket seats.