Removing rear window; 66 Valiant

One year only window & the seal is not available.
You could consider a sawzall & cut it out.
I did it once on a '66 Valiant convertible, it was a bit less cutting than the rear one you're talking about but I was able to save the seal. It wasn't fun.
But if the seal is bad then just cut the rubber seal with a razor as others have said.