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.
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1959 M35A2 LDT465-1D SOLD
1967 Dodge W200 B383, NP420/NP201 SOLD
1969 Dodge Polara 500 B383, A833 SOLD
1972 Ford F250 FE390, NP435/NP205 SOLD
1976 Chevy K20, 6.5L, NV4500/NP208 SOLD
1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8
RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.