After messing about trying to make a custom modpack, I've almost got it (I believe). Unfortunately, now when I attempt to launch the modpack, it appears to unpack the mods, and forge initializes, but after the Mojang screen flashes the screen merely becomes a gray screen, with no error messages. Any help with this would be lovely. I'll add some links.
Modpack: https://dl.dropbox.com/u/41610332/GrrxPack.zip
Technic Platform: http://www.technicpack.net/api/modpack/grrxpack