Is there even a true source of the 70mm 6 channel audio?
We have an in-theatre recording, but those aren’t really good enough to turn into a real soundtrack. They make fine references though.
Err, and here’s my “wrong movie” original answer (about Star Wars, not Empire):
The track we have now is assembled from “true enough” sources for my tastes. As hairy_hen said earlier, the 70mm mix was based on a four-channel master, which can mostly be gotten back to via the 35mm matrixed stereo. Then, we have an in-theatre recording of the 70mm audio which allows us to know the timing and relative volume of the differences from the 35mm version, minus the channel info. And we have SE audio which contains these differences, which can be mixed in at the appropriate places, to achieve a pretty accurate reconstruction of at least the four main channels of the 70mm mix. I’m not sure how h_h handled the LFE audio, but that may have involved a little more guesswork, adjusting and modifying the equivalent SE channels to create something more faithful to the original.