Stooby,
Since I am not there, it is hard to see exactly what you are seeing. But, I have to say, after installing the new door seals, in my 53 3100, they are considerably harder to get to shut. If your old ones were worn out, you probably got used to the way they shut. It is quite a difference between brand new ones and old ones. They do seem to get easier as the seals get pressed together, from the door being shut. Also, if the windows are up, it more difficult. I know that these trucks are far from air tight. But, letting the air escape out the windows, when you shut the doors, helps quite a bit. At least, it does with mine. Perhaps there are some more adjustments on a 56, than on a 53. If you want the door to fit flush with the cab, you just gotta, get aggressive shutting the doors for a while. Hope this helps. Maybe somebody will chime in, that has a 55-57. They very well may have a better answer for you.
Good luck.
Rugo
