If I may ask, what difficulties are there, really, in directly porting the pack up to 1.7.10 without changing any functionality, other than using lots of time? (Obviously, redoing the pack completely for an updated 1.7.10 experience would take much more work)
Pretty much all of the mods which were in the original have been updated, except (if I remember correctly) Rock Digger and Tinkers Steelworks (and, personally, I feel the pack would be better without Rock Digger--I have bad memories of having to build a high oven to get the last few seared bricks for a smeltery).
The only other two problems I can think of would be in redoing the HQM book, and redoing all of the structures to use 1.7.10-style blocks. The former would probably take a while (though, except for pneumaticcraft and AE2, could be mostly copied and pasted), but could probably be done while having youtube/TV running at the same time. Fixing the schematics would be a bit more of a hassle, but much of it could probably be done via a short computer program (Replace "<728>" with "exnihilo:dust" or something like that for each block type), especially since most of the schematics (from my brief reading of some of the ruins and city structure files) seem to share blocks.
There's also the Lockdown stuff (build spaceship), but I assume that would probably be relatively easy to implement, for someone who actually knows how to use them.
Secondary question: Would it be at all possible to set the world up as an endless sea of sand dunes, rather than just a flat world with occasional dunes?
(And if a 1.7.10 version does get up and running, please, please don't add agricraft
)
By the way, Crash Landing was/is absolutely amazing. It's the first modpack I ever played, and it's still my favorite. Really quick to get into (unlike sky blocks), an incredibly well balanced line of progression, designed well enough so that even near end game there're still those times in the cities where you think you're going to die, the constant struggle of water vs food vs mobs vs progression which forces you to juggle multiple tasks at the same time, and the perfect combination of mods which fit perfectly together (As mentioned earlier, Rock Digger does not fit the "perfect combination"). All in all, an extremely challenging, yet incredibly rewarding pack which has such a unique feel. It would be really sad to see it die in 1.6.4.