Mavimao said:

he didn't use any special voodoo, he simply just spliced in the 85 mix in those places

Ah, that makes sense. I thought that must have been it, but couldn't remember exactly. The only thing that would stop it from being a seamless transition is that the '77 version has the channels panned inwards by a fair amount (presumably this was to reduce crosstalk in the surround, since upmixing processors were less advanced in those days), while the '85 has the full stereo width, but presumably he was able to match this by ear using the pan controls on each channel.

I just had a big test today, but now that it's done with (I already know I aced it!), I'll be getting started on this very soon. ;)

As far as lossless tracks for the Bluray are concerned, if there only ends up being enough disc space for the three theatricals, that will be fine, since the '85 and '93 sound very similar to the stereo and 70mm versions anyway for the most part. High bitrate AC3 tracks should be sufficient for them in that case.