Get a whole triple tree setup for the forks you are using.
Yamaha was very consistant on the triple stem and you can either use the whole triple or press your stem out of your lower triple and put it in the newer lower triple (if it is longer/shorter). That should be about all you need to do. Stock bearings will work in either situation, but now would be a good time to upgrade to tapered rollers if you don't already have them (the YZ's did, but I have never played with an IT. Most of the DT's had ball bearings).