oil pressure switch hard to replace?
No not hard at all. I'm not sure about the the 1975 but I'll assume it's placed in a similar location on top of the oil pump under the sprocket cover. When mine went bad though it tripped at high engine temps and high revs, when the oil was thinned. I believe the light would stay on once the sensor is blown completely.
I'm wondering if your oil pump is going, due to not pumping well enough at low RPM but pumping well enough at higher RPM. Please wait to get other opinions on that diagnosis.