Removing rear window; 66 Valiant

Not familiar with that early but believe should be very similar.Here is pretty much what I did on 75 Duster. I think my daughter was around to assist, but could have done myself.
Get trim off (With a clip release tool best to not damage it) pull rubber lock strip if it has one. Cut the gasket lip over glass and remove from top and sides. leave bottom part of seal uncut to act like a hinge. Lay a moving blanket across the trunk. Get inside and slowly push one corner till glass moves out from the seal. (this was worst part for me. Did mine in July and it was like an oven inside) Don't get too crazy with it cause bottom might not be holding much and you could launch it if you push to hard. Work it across till top of glass is all free and part of the sides. Go back outside and continue working glass loose. Cut bottom of seal if needed or just pull the whole seal off the flange and leave it on to protect the glass. Do not chip an edge! Back glass should be safety glass that explodes into a million little nuggets. Not laminated like front.