Are you dealing with a nice new, soft rubber gasket, or an old hard one?
If you are reusing the one from the slider, might want to check 2 things, 1) some sliders are a little smaller the others, seems I remember Dodge used 2 different suppliers for the sliders, one was a little smaller then the other, the difference between the 2 sizes was made up in the gasket, if I remember correctly. The glass was the same size as the larger slider (maybe there were 2 different size rear windows ).
2) If the slider was aftermarket, the thickness of the rubber gasket flange may be thinner the the thickness of the glass.

At any rate, I do remember people having issues with glass/slider sizes being different in some cases. How is that for good news? Sorry. Gene