As other have posted, there are several seat belt suppliers. I bought mine from Seat-belts Plus. I installed them in my '46 Super Deluxe Coupe using 1/8x2x2 steel backing plates under the floor pan. SB+ sells mounting hardware. I removed the front, rear seats and flooring. The biggest part of the job was measuring and selecting hole locations. I didn't go to the frame. The front seat belt clips line up with the seat in the rearmost position. I only installed two sets front and rear.
https://www.seatbeltsplus.com/?gclid...SAAEgLSgfD_BwE