Classic/Antique Car Repair: 1973 Mustang Mach 1 - Oil Pressure Low, blown head gasket, mustang mach 1

Question
Oil pressure is below Low.  Thought I would change the oil, was about to fill, and the filler cap had white emulsion on the bottom of it.  

What is wrong?  How hard to fix?

Answer
The white emulsion indicates water in the oil. There are several possible sources. One is a blown head gasket. Another could be a plugged PCV system not venting condensation out of the crankcase. If the oil pressure is low it could indicate worm bearings in the engine or a weak oil pump.
Brad