66 valiant rear glass seal needed

Your only real option is to buy bulk windshield seal and make your own- some glass shops may have an old guy in back that remembers how to glue them up if you don't want to do it yourself.
You need to match up the profile of your old seal to a bulk (roll) seal and buy it by the foot.
Examples from RockAuto:
Body & Lamp Assembly > Weatherstrip > Windshield Molding / Seal | RockAuto