Are you getting the entire third (i.e gears and all) or just the carrier.

If it is the former, it is an easy swap. Although, you forgot the step of "stop, get hernia surgery when you pull out that 90# chunk of cast out of the axle." Of course, getting it out is easy, because of gravity, getting it back in is almost impossible. I would start praticing your close quarter, out of position bench press ASAP.

If it is just the carrier, I hope he sends you the races that go with the bearings. If not, you have some work ahead of you. Actually, because it is your rear axle and it is used daily (or at least on the road, at speed) you really need to have someone set up the gear pattern. The pinion should be fine, but you need preload and back lash set because that carrier, while close, is not exactly the same as the open one you have now.

Good Luck,
Michael