hairy_hen said:

I'm quite certain that the stereo mix was done first. My reason for saying this is because there a few sounds from the 70mm version that do not appear in the 35.

My understanding is that the Dolby Stereo and the 70mm 6-track mixes were both created from the same source elements ("mix stems"). The differences are just down to the final mix. For example, the explosion sound when Han takes out Vader's wingman in the trench is much deeper and panned hard left in the 70mm (and the '93) mix, but is still present in the 35mm, only much less noticeable.