I'm not sure MEC offers a short kit for their 12 gauge Sizemaster to change it from 2 3/4 inches to 2 1/2 inches, at least I didn't find one.
Simple solution is just make a 1/4 inch spacer to place under the shell at the Crimp Start station and Final Crimp station as illustrated below. And additional Rube Goldberg (an engineering term some of you may be familiar with) feature to add to that spacer is a rim guide to more precisely position to shell at that station and to keep the shell in place on the up stroke.
Jolly
