Well, I just did my '51. Second time. Cab seal around the doorway, correct? The first time took me a while. Then I became a smarter monkey. Hand soap or dish washing liquid is your friend. Lube the section that goes in the track, the flap and the bulb. Slide the front doorway section first. Then slide the rear doorway section. You'll need to pull the bottom portion in a downward motion while feeding at the top. Then, at the top, I tucked the lower channel part and while pressing, pulled back the flap and used a screwdriver to snap in the upper channel part. A little pressing with your thumb and it wants to snap in to place.
This is all assuming what your working with is similar to what I was working with.
