4 point seat belts or not?

Thinking about going the 4 point route in my 36 Chevy 2 door sedan and would like opinions on what would be good or bad about this option. There is no place to mount a 3 point system to the B pillar and I want more than just lap belts. I have not ruled out sebring type seats with the built in 3 point system but I have found a lot of bucket seats I like that would work with the universal 4 point systems. Why don't more people use 4 point belts in streetrods, what are the positives and negatives?
Thinking of these in all 4 seats to accommodate the wife and 2 kids. http://wescoperformance.stores.yahoo.net/am4point.html
Thanks

Jeff

The problem with the floor-mounted shoulder belts is the line of pull. In a head-on incident, the body will be thrown forward. Because of the anchor point at the floor, this will cause the shoulder belts to be pulled downward on the shoulders,resulting in crushing of the spinal vertebrae. The force can be such that the belts actually pull down through the seat.

The shoulder belt anchor point needs to be directly to the back as soon as the belt comes up over the shoulder. That's why the crossbar in a roll bar or roll cage installation. That's to anchor the shoulder belts. Problem is, most cruiser guys don't want to install a bar across the car because it inhibits entry/exit into the rear seat.

I have young guys and gals who show up all the time at Friday Night Drags technical inspection at Firebird with floor-mounted shoulder belts. I instruct them to go back to the pits and remove the shoulder belts all together, then come back to tech.

Here's a tutorial from SFI concerning belt installation....
http://www.sfifoundation.com/