does anyone know of an aftermarket interior kit, or substitute rear seat for a 1970 Mustang convertible that has a 3 point seat belt system?  I have two young boys (3 and 5) that like to ride in the drop top with me... they have outgrown the car seats that have shoulder restraints built in..  the next size up requires shoulder belts and the 70 only has the lap belt

any ideas?  

Might check with some of the street rod companies - lots of seat belt options in that arena.  You might have to fab mounts.  If I get a chance, I'll look at my copy of the 1970 assembly manual and see if there is anything mentioned about rear belt mounts.