On my 74 Euro Pantera, I recently removed an original side window for the same reasons you described. From inside the engine bay, you push the window out. It did not want to come out at first. Then I sprayed it with silicon lube and used a small hooked tool to loosen the rubber lip. It eventually it loosened up and came out just fine. I put the rubber and frame back in open just by hand. When I eventually put the glass back in, I will use parachute cord to pull the rubber lip in place.
