How do you know "coolant" (refrigerant) is fine? Is static pressure above 47 psi? If so, and relay is not energizing, then problem is most likely an input to the ECM is not happy. So the two primary inputs are the low pressure cycling (evap freeze protection) switch on accumulator, and the...

No codes get set for relay problems or air gap too wide issues on the C/K platform. Very, very common for wide air gap- I've done maybe 4 or 5 a season for that era truck. Remove a shim and get another 100k miles out of compressor.

Never seen a gauge with a kit measure the high side only---they measure low side, and when you say there is too much pressure- I can only assume this is with a/c off- since code 66 disables the compressor. and you are merely measuring static suction pressure which would be "in the red" if ...