Searched the clutch problems here and can't find something quite like this:
ACT HD clutch installed about 4K miles ago. Sometimes (and getting more frequent) when I depress the clutch peddle the clutch does not disengage, and there's resistance when I shift out of gear (stays stuck in gear). If I keep the clutch peddle down and I brake to a stop, the clutch will release before it stalls. The car only has about 50K on it, with the clutch replaced about 4000 mi ago. Thoughts?

 

Might just need to adjust the clutch or bleed the clutch fluid.

 

Sounds like something is binding. You may have the release point wrong on the master cylinder though. Try a bleed and adjust as mentioned. An extreme case would be a dislodged spring, but that would be unusual at 4k miles. Your heavier clutch may have finished of a weak master or slave cylinder too. Check both for leakage.

 

Check that the fork for the throw out bearing is seated on its pivot point properly as well.

@SgtB --> Is that your disc? I had this exact problem on mine a while back:

 

Yes it is. Exedy aye? I was a bit ....disappointed. I did get almost 60k out of it, and the pad had tons of life, but the plate appears to be too thin for my right foot. Know any clutch disks that bite like the exedy but hold up better?

You know what? I have no idea where that inner spring went on mine. I hope to god it fell out when I was pulling everything off and isn't behind my flywheel. Didn't even think about it until I saw your picture.

EDIT: Never mind. My disk is single sprung.