Have you tried putting the bike in fifth gear putting it on the centre-stand and rocking the rear wheel back and forth?
You might just break it loose that way...
When I bought my 1976 C-model, the engine was free but the clutch was seized as tight as a frozen turkey. I changed the oil, changed the clutch cable (the old one was nearly busted), lashed the clutch lever (which was totally frozen) to the handlebar with a bungee, put the bike in fifth gear and started doing the Fred Flintstone dance on the garage floor (wheels down obviously) in time to some rock and roll that was playing on the garage stereo.
Within about 20-30 minutes, I realized that I had migrated about 10-15 feet across the floor and there was no skid mark. I also noted that the clutch lever was much closer to the handlebar than it had been. After another few minutes, the clutch lever was contacting the handlebar and I could pretty much duck-walk around at-will. I propped the bike up on the centre-stand, sat down on my bum on the floor and started running the back wheel around. After another little while the clutch was completely free except for the usual amount of oil-drag.
Since then, aside from a top-end rebuild, I have put more than 11,000 miles on the bike and have not been inside the clutch basket at all.
Best of luck!
Pete