One thing to consider when thinking of adding seatbelts to an older vehicle is the design.
I'm not confident that just dropping a belt and shoulder seatbelt alone in an old truck makes you any safer.. and in some situations could probably do more harm than good.
Passenger restraint is more than just the belt.. it's the overall design of the passenger compartment, in conjunction with the belt that makes it safe.
For instance, in the 1950 1/2 ton I had, the bottom portion of the seat was pretty easy to slide forward. In a front end collision, there is no doubt in my mind that the seat would slide forward.
A '3 point' restraint system (lap and shoulder belt) would keep my body from moving forward, but it wouldn't prevent the seat my butt was on from sliding forward.
Other things to consider would be that most old truck steering columns do not collapse, so in a high speed front impact, you're getting shish-kabobbed weither you have a seatbelt on or not.
More modern vehicles have shock absorbing front and rear bumpers, so if you do get rear ended (or hit something head on) the bumper mounts absorb some degree if the impact. This not only helps reduce the damage to your vehicle, but it also helps keep your body from taking the full brunt of the impact against the seatbelt.
Glass is another concern.. most folks have plain old laminated glass.. that stuff is just regular house window glass with a layer of lamination on both sides.. when it breaks, it breaks in long jagged shards.
I find it interesting that so many folks, in the name of 'saftey' will go to great lengths and expense to move the fuel tank out of the cab, and put in seat belts, yet ignore the three largest dangers in one of those old trucks.. the steering column, the glass, and the 'c channel' frame.
I don't say that to pick on folks who move tanks and put in seatbelts.. I say it simply because it would be tragic.. even deadly to assume that just because you've got some 1978 Pinto seatbelts in your truck, and moved the gas tank to under the bed that it's now somehow 'safer'.
