Any hardware store with Hillman fasteners should have the rubber well nuts.
The stock R12 setups on the 73-87 trucks only have a low pressure cutoff switch LPCO on the accumulator. It should be the fitting pointing toward the blower fan. The other fitting should have a Schraeder valve and threads for R12 low side manifold gauges.
A high pressure cutoff switch HPCO, if someone added the fitting for it, should be on the compressor head or on the High Pressure hose fitting off the compressor.
A fitting further away from the compressor should be the Schraeder valve and fitting for the high side hose on your R12 manifold gauge set.
The LPCO and HPCO are wired in series between the AC control head in the dash and the compressor clutch.
If possible I usually get a Sanden compressor with the HPCO bung in the head or hoses for the Harrison A6 or A4 compressors with the HPCO bung installed.
I usually install a Snubber diode across the compressor clutch coil if it's not already present in the compressor plug... Even if you don't add a HPCO switch I would make sure a snubber diode is installed properly. It keeps high voltage spikes generated, when the electromagnet coil switches off, from backfeeding into the electrical system of the truck.
The easiest way to add one is the GM ATM fuse package AC diode... Littelfuse 02400104P 1A Mini Diode GM #12135037 and a Metripack 280 sealed ATM fuse holder.
You can buy the diodes and Metripack ATM fuse holders on Amazon.
Fuse holders can be purchased already terminated with wire pigtails for soldering into your wires or in kits with terminals, seals, housings, and cover and U provide the wire.
Attach one wire to the clutch power and one to the ground. Insert the diode with the Cathode arrow and stripe pointing toward the clutch power and the anode end (wide back of the arrow) to ground. This blocks current from flowing through the diode to ground but allows the high voltage AC inductive spike generated by cutting clutch electromagnet coil power to flow back into the clutch coil and quickly, and harmlessly, dissipate rather than backfeeding into the vehicle power system.
This video is a visual in-depth Electronics tech explanation on Snubber diodes and why to use them around Induction coils like relays and electromagnets. He shows scope traces of the inductive spikes and the way snubber diodes impact the voltage levels of the spikes using a test circuit on a breadboard. His relay is a much smaller electromagnet than the AC compressor clutch. I've seen over 500V spikes from AC clutches...