How hard is it to change the Oil Pressure Control Valve?
Any hints from someone who has done this with the motor installed?
Is this the most likely culprit causing my high oil pressure?

 

What is your oil pressure running? Are you getting the reading off of the stock gauge only, or have you confirmed excessively high pressure with an aftermaket gauge. The oil pressure relief valve and the oil pressure regulator, both require oil pan removal to access.

 

I have not tested with other than the stock guage that is in the dash, but it recently started reading high, e.g. around 100 at 55mph and 70 at idle - I am concerned that I might cause damage by running it with the high oil pressure

 

Its probably your oil pressure sending unit, the little brass mushroom looking thing under the oil filter, see if you can grab another one from a junkyard, or a new one. They go bad pretty easy, I've already replaced 2 on my car in the last 3 years. just take the wire off unscrew it and pop a new one, really easy. Thats definately what i'd do first, cause 100 PSI is alotstock sould be around 60-65 past 3k RPM.

 

About the only way your oil pressure is going to go up is if you have a lot of contamination in the system and the opr and control valve are sticking, very, very unlikely.

2nd vote for sending unit replacement.