Your stock may be too long if you just add a pad, you may need to cut your stock back about the thickness of the new pad.
True, IF the original LOP is desired.. However, on most 7400 stocks the cheekpiece starts rather close to the end, therefore if one is adding, say, a 1-inch pad - the cut might be 7/8" before final sanding/shaping and the pad may need to be bigger in order to fit near, or at, the rise of that cheekpiece.
They can be a bit of a pita, but overall certainly doable..