http://www.minecraftforum.net/topic...s-guns-multiplayer-tdm-conquest/#Installation
So, as you already have most of the steps done with FTB directly, you only need to download the mod, add it via the button "Edit Mod Pack".
Launch the game once, it should crash, but it will create the config file you need.
Now, go to the pack folder (where you installed FTB launcher), and find the "config" folder.
In there, there will be a config file for the mod, where you can change the 3 block IDs used to something unused, for example 550 551 and 552.
Now it should work, but you might have to change the item ids if they happen to overwrite something, the process is the same.
As a side note, NEI has a function that dumps all unused IDs to a file, you can do this before putting the mod, in order to find IDs to put the blocks at.
Hope it helps.