68 Cuda Rear window seal issues

If this is a 67 body that is getting a 68 gasket, you would need to trim the length of the "fence" the gasket sits on. I learned that a long time ago. And that was while reusing OEM glass and gasket from a 68 on a 67.
If it is a 68/69 body, maybe too much sealant in the bottom is holding the glass and gasket upwards. If that's the case, and the sealant hasn't hardened, effort to push the top of the glass and gasket downward and into the car will roll it on into position. Some sealants respond to heat so a heat gun, along with pressure, can help.