Baby seat in my dart???

Ok guys, my baby will be here in 4 weeks! Now I need to put a baby seat in the back of my 69 Dart. When I got the car there were not seat belts in the car... and there still isnt any. What is the best way to put a baby seat in the back on an A body?? Will a factory lap belt be sufficient?

Thanks guys!

Child Passenger Safety Requirements