I have four pairs of Daltons, two of them shell. I find the fit difference between the calf and the shell versions to be nil. One of the shell Daltons is unlined (walnut) and other than the obvious differences in feel between lined/unlined, still no significant difference for me in fit.
I can't speak to how it compares to the Leeds or other 1-511 last shoes as I own none. But I do own Modified last, Barrie last and Leydon last Alden boots. I'm an 8D in the Daltons, 7.5D in the Modifed and Barrie lasts, and 8E in the Leydon, although I only have the unlined chukka in the Leydon so don't feel really qualified to compare it to the Leydon.
Others seem to be able to describe differences in last fit much better than I think I can feel from wearing them. Having said that, I do feel that all the above sizes in the various lasts do fit me very well. From solely a fit perspective, my favorite is the Modified, followed by the AE 1-511 last.
Good luck! What color shell are you considering?
This helps a great deal. Much thanks my friend! I'm considering the walnut shell.