Instead 6GH8A will be possible ECF-82/6U8A

They are very similar but not identical and the amps will more than likely sound different - but should work with no problem (no guarantees here). The 6GH8A's were not the kit "originals", the 7199's were; but because they became scarce the sockets were rewired to accept the 6GH8A. These tubes are still available and are IMO the best driver tubes for the st-70.

I think the ECF82 is a direct replacement for the 6GH8A. A Sonic Frontiers dealer sold me a pair of Telefunken ECF82 to use in a Line 3 preamp in place of the 6GH8A. They worked fine and were a good improvement over the stock tubes.