Go into the game (with all mods loaded except Flans), open your inventory and go to the NEI options screen. There you can create a dump of all IDs currently in use.
Then you open the config file of the Flans mod. Look at the sections where the IDs are defined. You'll be easily able to see where the numbers are matching some from the dump you created. You need to change the numbers for Flans mod to something different, something that doesn't match anything in the dump. Never change the numbers for an already existing mod, always change the one you're adding new!
Note that it may take some experimenting - there's both block IDs and item IDs, which are assigned in different number ranges (blocks go from 1 to 4,096 I think? And items from 4,097 to 32,768? I'm not sure myself). Also, some mods may require blocks used during terrain generation to be no higher than 256. Finally, sometimes there's an offset of 256 between the numbers in the config files and the numbers actually seen ingame (don't ask me why, I don't know).
If you want to be sure that it really is an ID conflict before jumping through all of these hoops, you need to analyze the crashlog.
...and now imagine doing this for 50-60 mods at once. You'll suddenly understand just how much the FTB people are doing for you in creating the modpacks
