Seatbelts were the first thing I did to my truck or I could not take passengers, my kids especially. The installation was easier than I expected.
www.seatbeltsplus.com sells a full kit for installation in my 54 truck and other similar years. Worked great although the weak link is admittedly the connection / gauge of the metal the shoulder strap is bolted into (even with an inner plate). The plate helps spread the stress through the metal but I'm not looking to test the set-up anytime soon.
