I'm with you RustyRod. I have been a paramedic for 16 years and have worked on countless critical MVA's involving all kinds of different vehicles. It's true that older vehicles are not built to todays safety standards. They do not absorb impact energy nearly as well, thus a much greater amount of that energy makes it to the passenger compartment, potentially hurting you. And that is the reason you should install and wear them.
There are 3 impacts in a collision:
1) The impact of the vehicle to whatever it's hitting.
2) The impact of your body to the interior of the passenger compartment.
3) The impact of your internal organs, including your brain,against the inside of your body.
Any increase in 2&3 above will multiply the injuries you receive. Reducing your movement in the passenger area in an accident is the whole idea of wearing seat belts. This is what saves lives! Are there isolated incidents where seat belts contributed to injuries? Yes. But, they are few and far between.
A great amount of thought needs to go into how you install them, though. As fumblin46 suggests, your attachment points are critical and movement of the seat in relation to those need to well though out.
This is what I'm gonna do with the 57 I just bought. Regardless of what the law says I can or don't have to do, I will install lap and shoulder harnesses, strenghten the weak areas and use them.
My 2cents.
