The original release of "Star Wars" in 1977 used 6 magnetic tracks on the 70mm prints. Those six tracks were left, center, right, surround, and two low frequency effect channels (below 200hz). Since our home theater systems are designed for only one low frequecy effects channel, Dolby Digital 4.1 is the best solution. The two low frequency channels would need to be combined to the .1, and the LCRS channels would remain original. Going to 5.1 would either involve duplicating the mono surround to both left and right surround channels, which is simply redundant data (which might require additional compression, lowering the quality on the DVD), or having a sound engineer create a new mix for the surround channels, which would not be the way it was presented originally.
Dolby 70mm 6-track with stereo surrounds was first used on "Apocalypse Now," released in 1979. I'm not sure if either "The Empire Strikes Back" or "Return of the Jedi" used the stereo surround setup. If they did, then a Dolby Digital 5.1 track would be appropriate for them.
"Tora, Tora, Tora" was released on DVD in Dolby Digital 4.1.