I have had good luck with my used 3650 in my S197 track car.

The 3550/TKO in the CMC car shifts terrible compared to a T5. The mechanism in the 3550/TKO is complete junk. They use massive shift lugs with a very sloppy mechanism. I guess a plus side is that it probably won't wear out since their all iron. A common mod is to blue print these parts to tighten them up. Definitely setup for drag racers bang shifting the thing, which was a point of failure in drag race application T5s.

The 3rd gear on the front of the output shaft is what always breaks in the T5 and TKO boxes (for me at least). I think as the box heats up and and becomes longer. The shafts can move and bend more reducing tooth engagement. Enough cycles like that and it'll pop some teeth off. Real race application transmissions have a little dog-bone thing that joins the two shafts together so they can't spread as easily. I did split an output shaft in a T5Z at hallett three years ago and it ended up splitting the entire case.

Jerry, is your drive shaft in good shape? Vibration through the output shaft of the trans could cause issues. The output shaft is also very long, making it more susceptible to vibrations from 'low speed' things like the drive shaft.

- Josh