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 ...