My 1976 VW beetle has the stanchions/bolt holes for rear shoulder belts. 

I've thought of installing some,  

mainly for the style i drive my baja *evil grin

Tom,

Studies have shown that having lap belts only can do more harm then not wearing a seatbelt, even
for rear seat passengers. If the lap belt does not fit the passenger correctly, or is out of position it can cause major internal damage. Shoulder belts are designed to prevent this, that is why they have been standard equipment on US cars since around 1990. The disgusting part is that the government required the automobile manufacturers to install the upper belt anchors in the early 70's, yet did not require the belts themselves for for another 15-20 years.