Steve,
A rear head for the SD7H15 Sanden compressor with R12 charge ports should still be available. Hose ports will be o-ring. See pic of mine below.
The SD7H15 is a 7 cylinder pump, whereas the 508 is 5 cylinders.
As you noted, the SD7H15 has slightly more displacement than the 508, which should provide slightly more cooling at low speeds.
John
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