Best case scenario:
Soak in PB blaster, use top-of-the-line freewheel remover to yank off.

Worst case scenario:
Remove freewheel by removing cogs from outer body (no sense in wasting the cogs, as they do thread on - any in good condition are quite hard to find, and good trade bait here), unscrew dust cap, remove outer body, remove pawls, then place outer body in vise along with rest of wheel. Spin against threading to loosen.

-Kurt