Unlock frozen engine

If that cylinder is really rusty, fill it with Evaporust first & let it soak for a couple days, then dump that & repeat with fresh for another couple days. Blow it out then put MMO or Stabil Fogging oil in there, let soak, add a little compressed air like You're leak-down testing. Chances are all the cyls. are compromised some, I'd do 'em all. I had to do all 6 on the 170 I pulled out of a '62 Valli that hadn't turned in decades.
Hopefully You won't have to pull the heads at this point, but You may, it's just a fact of life with these things sometimes. Valves could be seized in guides, soak them & tap the rockers with hammer & block to see if they bounce/move. Good luck with it!!