I'd call Special Projects MotorSports and talk to Kiwi about his Gear-X units. Pretty easy to deal with, and a good price. I've seen several of these units installed in IT tranny's, and no issues that I know of so far. http://www.specialprojectsms.com/index.php?categoryID=6
I have a Gear-X final drive from Special Project Motorsports in my YS1 and have had no troubles. Kiwi is a good guy and helped me quite a bit when I was looking for gears.