Reattach Window Glass

 Every door glass has a few adjustment bolts/hardware. If the glass has slipped out of the window channel, then taking a few things loose will be the only way to reinstall the glass. Usually you can loosen or remove the bolts that hold the window channel in place and move it around and back onto the glass.  Sometimes I have had to loosen almost everything, the window channel bolts the window regulator bolts and/or all of the window adjustment bolts.

The window channels can bend/twist easy, so forcing a window out or back into the channel can cause additional problems. 

As you have already stated the window channel (run/felt/liner) can also cause a problem. Yes it's old, brittle and old rubber/felt parts tend to decay with time. They will harden, crack and start to fall apart. With that said, Pandora's box is open, now that you are into the door/glass, I would check everything for proper fit and function. As with most things with old cars, one thing always lead into another.

  If your rubber/felt parts are all in decent shape, that would be very good for you. Unless you have a auto collision/restoration buddy to help you, you really need the TSM. (Technical Service Manual). Study the schematics for comprehension and understanding on exactly how the factory made the parts, the part(s) location and how to adjust/install the glass & associated parts. 

Lastly, many times the grease/lube from the factory has dried onto those parts, especially the window regulator. If the regulator is stiff and/or does not move smoothly this can cause numerous issues, including tinting the glass and forcing the glass out of the window channel. Clean all of the regulator, the arms, roller cams at the ends of the regulator arms and the metal channels that the roller cams slide back and forth within

There is also a specific sequence of how to disassemble a door glass and reinstallation. 

There are some other little things to discuss about, but I just gave you the main points. Hope my 2 cents worth helps!