The Stovebolt Site Forums The Shop Area Driveline recommended ATF Dextron three brands for 54 Hydramatic

That would be Dexron? Although, I have seen pros call it Dextron (maybe there was/is a non-GM product with that name?)

Hydramatic/Hydra-Matic

"2016 - DEXRON III(K) for Manual Transmissions
On August 1, 2016, GM released the Dexron III (K) fluid specification (GM Spec GMW17639) as a fluid to support older GM manual transmissions and power steering systems requiring the previously discontinued Dexron-III(H) fluid."

"Dexron VI fully replaces Dexron III and prior evolutions of the Dexron fluids. GM manufactured the liquid for compatibility with transmissions requiring earlier versions of the fluid. Dexron VI works in any vehicle that requires ATF-A (the original automatic transmission fluid) or a Dexron variation. GM's newest vehicles require the use of Dexron VI in the transmissions, however."