Well, the 1.4.2 pack didn't use the "universal" block ids they are now using for the various packs the FTB team makes, so you would have to change a lot of configs, sadly.
The main factor is that the pack for 1.4.2 was a beta one, and as said for the previous question, the block ids changes, so it is not just wordlgen. Note that you could take your old config, the new one, and work out the differences to make all work fine together, but I assume it could be quite a work, depending on what IDs changed.
Once the IDs problems are solved, you could use Mystcraft to get the new worldgen, yes.
Be advised that by default, the "Mindcrack" pack doesn't have mystcraft, so do take it from the "direwolf20" pack.