If you really want a trick project you could install 3 point seat belts froma late model vehicle. This guy
http://www.472ci-67mustang.com/Seat_...elts_index.htm used Hyundai 3 point system belts in his 67.
Since you will be transporting a youngun around you might want to consider placing some sort of firewall behind the backseat to block of the trunk area from the passenger compartment, or at least reinforce the trunk floor with something like this.
Tankarmor.com
In case of a severe rear end collision either one of these could make a difference. Any little bit helps with so many stupid careless drivers out there.