The mec jr is what you want for loading different lengths of shells. And once you get the hang of using it, you can plug along pretty quickly.

A piece of info on reloading RST (chedite) hulls, thry dont really reload very well. You maybe get a couple of rounds out of them. And you have to lifhten up on your crimp pressure i find to prevent the hulls from buckling. The high brass variants of these hulls seem to be better in this area than the others.
And the hulls seem to expand after fired once. I have had trouble in some of my ejector guns with chedite reloads.

A 2.75 shell in a 2.5 chamber will only raise pressure about 500psi. At the pressures most of us are reloading, that much wouldn't make any difference. Save yourself some time and money in the long run and just buy a Mec Jr. I but one brand of wads [ Claybuster ] and have them in 3/4, 7/8, 1, 1 1/8, and 1 3/8oz. The big difference is cushion height. You could use 2 3/4" shell data in a 2 1/2" shell and sub in a 1 3/8 wad with 7/8oz or 1oz of shot and use it in a 2 1/2" shell. You're giving up a 1/4" and that's one way of making it up. Bill had some good data. Let us know how things work out.