My input, if it can help...

If you are melting sources and looks you can't have a perfect match, just make sure the waveform is not inverted from a source to another.

Another thought, concerning the kind of mixes you're all using around here, is that the wide stereo surely means to make a nice experience for all people who have special equipment... However I remixed (and downmixed) myself the "Asteroid Field" stereo and mono tracks from Star Wars Musical Journey and I can tell you Shawn Murhy's mix sounded awful next to the final 2-track version I finally made - only Msycamore here has heard the result, though I didn't let him compare with the downmixed untouched version. What I had recovered showed the overall sonority of the RSO track, with a lower dynamic range and with compression artifacts they made for the DVD unfortunately obvious in the highest peaks.

What I'd like to point out anyway is that the stereo widening for 5.1 experience sounds totally unatural to me as a 2-track stereo shape. To obtain the best version I could, close to any musical experience for Hi-Fi ect..., I had to narrow the stereo with a stereo widening filter: about 30% for the rear stereo, 70% for the front stereo channels (100% beeing no change) + of course re-equalised the two pairs of tracks + the main center channel. The main consequence that really stroke me listening to the result is how much you can restore the dynamic range when you give back a more natural stereo by narrowing it.

Now, if I had to remix such a material I would try to give the wideness feeling through equalizations, and would perhaps even try to give the more relief as possible through two complementary EQ's, more bass-oriented for the front stereo, and highs for the rear... Don't know, but I guess

I strongly suspect this way of feeling and thinking the sound can make you reconsider the 5.1 phenomenon.

I'm not an audio "expert" though ;)... Just use the Force !