If possible adjust the door so the beltline is aligned at the rear . That would raise the door at the rear opening and increase the gap at the lower front of the door/door opening . Once the belt line is aligned then take next steps after re-examining the door opening. See attached pdf . Can you post a pic of the front of the door beltline ?
I saw what you were saying about the belt line. I missed that. So I went after that gap and had great success. Now at this point my gap is parallel around the door and the belt line is right on the money. I then drilled a 1/8" hole in the top and bottom hinge arm drilling through the inner door skin and completely through the arm. When I reinstall the door those holes and 1/8" rod will get me close to its permanent location. Now for the fun part, the door is to big. I don't know why but perhaps the gap was tighter in 1955.1 or GM used thin coats of primer and paint, or both, I don't know. Anyway, after epoxy primer, high build primer and paint there would be no room to open and close the door safely. Take a look at the pictures to see what I did. In a nut shell I ground back the latch side of the door 3/16" and in doing so the skins bend around lip fell off. Then I added back a new piece of metal to replace the fallen lip. Leaving the new piece wider then necessary I built on top of the new lip and in turn adding to the end of the door. My experiment was to complete 5" at the bottom of the door and see what happens. To my surprise I not only cut through the added metal but I also cut the weld.
