No, you can't "feed" the new one on. Too tight. Remove a foot or so at the bottom side. Compare it to the new seal and see that it will attach the same way.
I removed the old seal then started to install the new one beginning at the center of the top and working across and down one side and then the other. Make sure you buy enough. you don't want to piece it. Was around $200 at dealer. Probably cheaper elsewhere but as I said, wanted to sure of the size. Mine failed the same way. The metal parts rusted off and the seal would fall off the frame.
2016 F-250 Gas
2015 Rockwood 2608WS