It's really very simple, the 400 has a lower deck and therefore a shorter piston which allows it to fit within the taller 440 deck height......when the stroke grows from 3.75" to 4.15" and the rod stays the same length the piston has to get a bit shorter (typically 1/2" the stroke length) for everything to fit.

This is really the beauty of the 4.15, aside from the crank itself you really could do it (if you wanted to) on the cheap using all readily available oem parts. You just got to know the recipe.