So I went about this a little differently. I have a 67 barracuda fastback, and there are no provisions for a shoulder belt in the roof structure. I installed a set of seats out of a 2004 BMW M3 convertible. They have the shoulder belt integrated into the seat, they flip forward to allow back seat access and they have very good bolstering for staying in the seat during hard cornering. To accomodate the additional loading from the seat belt and to have a solid "butt report", I made a frame under the car with 2" C-channel and welded that to the subframe connectors. The seat bolts thru the floor to the frame with 5 grade 8 bolts per side, the extra bolt is under the rear corner that the shoulder belt is anchored at. The downside to all of this is that the seats themselves are extremely heavy, and then the frame under the car is additional weight, but this is a compromise I am happy to live with. When I bought my car it didn't have any seats so I would have had to purchase seats anyway. I paid $400 for the seats, an additional $200 for shipping them and roughly $35 for the steel and hardware for the frame under the car