A/C bulkead 12AN to sandeen 10AN

A photo to show the problem you are trying to solve would surely help. Perhaps you could have a hose made w/ a #12 fitting on one end and a #10 on the other end. Find out if truely AN flare fittings. Most after-market AC hoses are O-ring type. Old under-dash units used SAE flare (different than AN).