That would explain why they got the DVD 5.1 from the 6 track masters if indeed they did for Raiders.
Darn this can get confusing for trying to keep which film had which mix straight.
On Alien, you should be good with the ProLogic PCM. Just give the tracks a listen and see which sounds better, fuller, and more impacting by ear. I've found that these tracks can sound great even for modern 5.1 mixed films. I don't have a demodulator but I've gone back and forth with well mixed DVDs and come to the conclusion that sometimes I just like the clearer but less surround defined 2.0 matrixed PCM versus a well mixed lossy 5.1.