From a previous post, if you tried revving the engine and it started smoking the belt, you've got far larger issues than the belt or the driven clutch. As far as troubleshooting goes, the clutch doesn't care if it's completely open or completely closed on the belt in order to spin the shaft. If the shaft simply doesn't turn in neutral, or turns with any difficulty, the driven clutch has nothing to do with it.

Take the belt off completely, and spin the transmission by hand using the clutch. Ignore the fact that the clutch is on there, and just see what it does/doesn't do on each side of the machine in forward/neutral/reverse. Block the machine up in the air so you can actually see things turning as you go. Check the end play on the main shaft, too. A little bit (1/8" or so?) isn't bad, but if you've got a bunch of in and out slop on the shaft, you may need to address it. It's a good idea to get the new belt anyway, but by the looks and sounds of things, the T-20 should come out. I know it sucks; I know it's a lot of work.