"But I don't feel that the lap belts that are in my 87 & 86 vans are so
unsafe that they can't ride in it.  They might not be perfect, but they are
a lot better than none at all."

I can tell you this much, a lap belt "only" is very unsafe in the back seat. What happens is when you have a front on collision the body folds all the way down to the knees or further and can very likely cause permanent spinal injuries. If you are in the front you would hit the dash and this would keep you from folding to the knees. I have seen studies done on 60 minutes or 20/20 or one off those shows that determined it was safer to not wear a lap belt in the back seat. I'm not telling anyone not to wear a rear lap belt, that's your decision. Just food for thought. Personally, I never wear a lap belt alone. 

Chip