AC Pressure Switch ?

[hatzie]

R12 usually only had a LPCO.
Several AC hose and fitting vendors have liquid side fittings with Schraeder valves inside so the switches can be changed without evacuating the system.
You may be able to get a head for the Sanden 508 that has provision for a HPCO switch... if not you can get braze in sections for the hard lines with an HPCO fitting as described above. The latter is the route I took. HPCO is installed near the high side gauge port on my 76.

You definitely can get a 7 piston Sanden 508 compressor head machined for the same line fittings as the 1983 Harrison R4. https://www.ebay.com/itm/Dirty-Dingo...-/191979228686

The Third Gen F-Body guys have a lot of info on this kind of conversion.
https://www.thirdgen.org/forums/cool...e-systems.html

If you want an engineers perspective look at the E38 BMW boards... the Corvette boards are a friendlier crowd and full of engineers too.