I have a 96 Subaru Legacy L and recently installed an exedy clutch and a new flywheel and now the clutch chatters bad out of first gear. One mechanic told me it's because I used an aftermarket clutch and the other told me he'd have to take it apart to know what's happening. Anybody know what's causing this? Also, if I pull the motor what are some things for me to look for that might be the culprit? I'm guessing I'll just need to put a whole new clutch in and flywheel

One possibility:

Did the clutch release bearing (I'm assuming that you installed a new one) slide freely on the input-shaft for the transmission?

If not, then the release-bearing might get caught-up on the shaft, and not allow a smooth clutch operation.