Back glass install frustration

What is this?
A fastback Barracuda?
I had a problem on a fast back B Body.
Could see daylight at the lower left corner between body and rubber.
I think it is just not a good of a rubber fit as OE back in the day.
I suspect that is your problem.
Poor rubber fit.
I've that problem on the front of cars as well.
Poor fit.
(I used polyurethane sealer to fill it, but my interior was out.)
If you have to remove, cut the gasket.
It's usually cheaper than glass.