Here are Don's tunings as written down by the man himself.
Of course Don's E6 isn't actually an E6 chord because there is an A in it.
However, Don pretty much only used the top 6 strings of that neck (which was an E6 chord) and used them on pretty much all his solos. Apparently he only used the other (B13) neck for a few fills here and there.
I think Don's "E6" is probably best thought of as 2 tunings on one neck - an E6 on the top 6 strings and abig fat A major on the bottom 3.
Also Don rarely (if ever) used bar slants so that should give some more clues in working out what he was playing.
That 6 string E6 is actually exactly the same relative tuning as C6 (ECAGEC high to low) moved up 4 frets so you can play most of Don's stuff on a 6 string C6 lap steel.
The key to Don's sound though is that 6th note, so pretty much any 6th tuning will get you in the ballpark. I really like A6 and that is what I consider to be my primary tuning.