Rear seat belts

Has anybody ever put rear lap belts in an early B-body? I put front ones in, and there were already places in the floor to bolt them in, but I don't think the rear has anything. Where would I get them and what do you bolt them to?

I want to take my son with me to car shows.

I have the exact same thing going on in my 64 Fury.
Were rear belts even an option?