I've seen slip clutches of all brands seized from new. Symptoms are early driveline and gearbox failures. I know your baler is long gone, so I'm just putting this out in case others are experiencing similar problems. A basic diagnostic is to feel the slip clutch after an hour. It should be warm from slipping a bit on each plunger stroke. It's possible yours was working correctly. Something else could have been wrong, but this is where I would start.

I put some 150,000 bales through a 346 when I was working as a hired hand. I was able to pump 6" cakes (6 strokes per 36" bale) all day long in crappy windrows with lots of slugs. I couldn't believe the manual said 12 to 18 cakes per bale. That seemed unreasonably slow. Who had time for that?

The flywheel shearbolt used to fail at times, but usually under light or no load. I learned since it was likely a worn bushing at the flywheel. The driveline never failed despite also running the hydraulic pump for the kicker. The baler tongue did from the constant up and down jerking from the kicker. The resisters in the bale chamber wore out. The symptom was it would not tie properly if I pushed it any harder than the 6" cakes.