Thanks guys. After a few nights of adding oil to the plug holes and a little breaker-bar persuasion, the engine broke free.
I'll now follow the steps BarryA laid out and let you know how it goes: " drain the oil, replace the filter, remove the valve cover and pour oil all over the top of the cylinder head, finish filling it with oil, and crank it with the plugs out until the oil pressure light goes out."