I never realized that pre 1990's had no shoulder belt. That sucks.
I would think with a forward facing the lap belt only would work, since I think the seat in my explorer has the lap and shoulder belt through the same lower hole on the frame of the child seat. Just remember there is a special way to put a seat belt on a child seat in a non latch style attachment.
You have to hook it up, pull the belt all the way out and then let it retract and it will click click click.
I catch flack from my wife, my parents, my inlaws etc. about the idea of my kids in my car. I win most battles, I may have to let this one go or maybe install a 4 pt to make everyone happy.