I am pretty sure that the AC-3 track on Alien is a straight transfer of the 70mm mix and not a new mix. It was a selling point on the LD when it came out — granted, there wouldn't be split-surrounds, but the front separation (which was quite significant in this movie), discrete surround and dedicated bass channel would have made it worthwhile.

Regarding your grading for Aliens: I like where you're going, but I think there is just a tad too much red right now. That said, it's amazing how much more vibrant the image can be without the cold pall on the Blu-ray… the two-shot of Hicks and Apone is a perfect example.