One thing to be aware of is the fall in aircon performance when changing from R12 to R134a. R12 is a brilliant refrigerant even if it does destroy the ozone layer! Hence, it might be worth considering filling the system with one of the better (more expensive) replacement gasses available rather than the usual R134a. Replacement compressors are freely available, either direct replacements, lighter replacements in aluminium or kits to fit modern compressors (typically but not exclusively from the US). If using a direct replacement GM Harrison compressor and the bolts don't screw in easily, it's as well to note that early casings had imperial threads for the mountings and later versions were metric. If you can't find replacement hoses from the usual suppliers, it should be possible to have hoses made up by a hose specialist using the original ends. Mike.
2014 XK Dynamic R convertible; 1977 Daimler Double Six Coupe; XE S