I've been reading the thread about the '77 stereo mix in digital, but I haven't actually heard it (if I am to do so, it would have to be posted somewhere other than newsgroups). It is definitely a great discovery for the sound quality of that mix, though I don't think it will affect anything I'm doing for the 70mm project.
My main reason for wanting to use the '85 mix in my edits this time instead of the '77 was that its wider stereo image matches that of the '93 mix—in the previous version the reduction in width was (to me, at least) easily detectable at times, and limited my options in choosing where to switch between sources and disguise the transition. In the new version I've had more freedom in this regard, and often even I can hardly hear the switch once the EQ has been matched. Some of the most troublesome edits may only be given away by something as subtle as a change in the tape hiss.
Occasionally I have felt that the original stereo mix could be a bit more robust-sounding than the 1985 in brief parts, so I guess I haven't completely ruled out using it a little bit; but if possible it would be better not to, if this discrepancy can be otherwise accounted for.
As a word of warning/disclosure, sharp-eyed viewers may notice that the new 70mm track will briefly drop synch about halfway through the movie (when the TIE fighter sneaks up on the Millennium Falcon at the ruins of Alderaan) before regaining it a few seconds later. This is because the '85 mix ran longer than the '93 here for some reason, and I couldn't get an exact match without making an audible cut in the middle of dialogue and noticeable sound effects. I haven't yet tried muxing it to see what it looks like, but since the delay amounts to less than one frame and is of short duration, I don't think it will be much of a problem. Of course, if I do find a way to fix it without the delay or making a jump-cut, then I will certainly do so.
Earlier I spoke of finding two sound effects in the surround channel which I couldn't positively identify as being 1993 additions or authentic 70mm originals, but just to be safe I deleted them from the track, since the former case seems more likely. I also found another very subtle effect that was added to the '93 mix and hasn't been pointed out before: a tiny little rumbling sound while the rebels are waiting for the stormtroopers to board the ship, which I think is also in the mono mix but definitely absent from the in-theatre 70mm recording. It's so quiet I'm not surprised no one ever noticed it before, but for such a tiny addition it sure did take a lot of work to eliminate. This addition corresponds to where the special edition inserted some very loud effects of the ship being pulled in overhead by the Star Destroyer, and the SE version may actually include this as part of it.