[xcntrk]

This question should be easy to answer by those running an emaxx tranny in a revo.

But if not; you can use a small wooden towel and begin mocking-up the necessary size that fits between the drive-cups. Once you have a trimmed down template, measure it with some precision (caliper) and begin searching for similar sized center shafts. I too am doing this process now for a custom project. It sucks, but the dowl will cut down on trial & error.