I just talk to a seasoned pro and he said Dex 3 will work just fine and that 54 Hydra-Matic isn't that fussy, you could pee in it and it would run just fine. You can use Dex 6 but it cost twice as much as Dex 3. You can still find Dex 3. The automatic transmissions in the forty's and back used whale oil, those are a little more fussier. He wouldn't guarantee the tranny with the pee though...
