Patrick, do you have a photo of the hatch in the closed position? Have you tried to lubricate the hinges? They may be dry and causing some of the binding that is causing it not to close. You may also try to remove the lift gate supports and remove the screws that attach the lift gate to the hinges, Try and close the lift gate now.
If you remove the hinges, take some pictures of how they are mounted to the header and a good picture of the hinges themselves. I need to see what hinges were used in 1949, I have hinges from the later 1951 and I know they are different.
