Its kind of common for clutches to rust to the flywheel in high humidity. We had about a dozen Cummins pullout engines in a storage trailer last winter which was the wetest winter I can ever remember. I think every one rusted the clutch to the flywheel. Of course its not a big deal with an engine on a stand. I got one unstuck years ago by putting a broomstick on the clutch pedal to hold it down and then I took the bottom cover off and hit the disc with a center punch in the gap between the linings. Hope something like that works so you don't have to pull the transmission.