infant car seat for 69 Mustang
I used the kind with the base that stays in the car and the car seat that just clicks in and out of the base. Really, any of them that use only the lap belt to hold it in place (which is most of them) will work. Where it gets tricky is when they get older and need a shoulder belt.
It's almost easier with an old car because you can get that lap belt so much tighter around the base.