Thanks so much!
Yeah, I made sure to tap the holes for the set screws and antiseize them, I wondered if Loctite would be a good idea for those nuts.
I didn't think I'd have time to tear it back off tonight, but my daughter wanted to hang with me in the garage, so we took the front clip off and took the PTO back off.
Here's what I found:
That piece was rubbing on the inner race of the bearing on the crank. Either I had the hub on too far back (don't think that's possible, think that clip prevents it from doing so), or that piece was pulled in too far by overtightening the clutch. The picture kinda makes it look like it rubbed through, but it didn't. The piece is fine, but I gotta get it back on there right next time! Now I can see why I only had a very short range of motion that would allow the clutch to disengage.
That's one way to learn that tighter isn't always better....