Congradulations on your Sizemaster purchase. I'm sure you will like it. I know I like mine. I use WAA41 wads for my 3" shells. Be careful if you buy new winchester 410 shells that say HS on them. These are a new design and require winchester's new 410 wads that are red in color, not white like the original ones. I only reload winchesters so I can't comment on the other brands but I know the WAA41 wads are used in them for 3" also. You can do 3" shells on the same machine you have and I find that it is really not that difficult if you follow mec's directions. I have dies for my sizemaster and I find I can change to a new gauge in about 10 minutes. When changing to 3" 410 shells, You have to:
1. Adjust the collum. The bolt goes in the top holes.
2. Add a 3"spacer to the deprimer.
Move the sizer collet nut to the lower hole.
3. Add a spacer under the primer cup.
4. Change the wad guide to the 4103 one.
5. Swap final crimper. I find this the hardest part to do, but if you remember how many theads are showning above the plate for the
410 2 1/2" you can save this for future reference when changing back again. The 3" crimper has the roller guide pin in the other hole and uses the nut above the plate instead of below like the 2 1/2" crimper. You put the crimper up there with the ram and nut from the other crimper. Thread the ram into the plate and the nut with nut on top. I like to leave about 2 threads or more showing above the nut, but 2 threads above the nut is where mine is set.
Remember to change the shot and powder bar and bushing. I use the universal adjustable bar on mine so all I have to do is turn some knobs.
To swap back to 2 1/2" shells, just reverse the order.
I hope this helps out and it will take at least one or two swappings to get the hang of it. Post some more questions if you need help. Mec's directions are the best place to start for changing to 3".
Happy reloading.
HammerMan