I was seriously considering Classic Auto Air (located in the Tampa, Florida area). They will rebuild your heater box (price dependent on what you need) and make sure everything operates and holds pressure as it should. They also sell the underhood items as a kit separately (includes their Sanded compressor). By going this route you won't have to carve up your firewall, and except for the compressor everything looks stock (I haven't talked to them about the compressor, but they may have a rebuilt factory compressor for your application).
Worth looking into.
fantum