You might check with a locale gunsmith and ask about apprentice work then move on once you've got the experience. You will want the FFL before doing anything for anyone else, trust me.
The ATF does say that if you are working on "Gun Smithing" on any firearm other than your own you must have a FFL. I don't have the reg.'s in front of me right now, but thats what it states clearly. If you are assembling "building" them then you must also have a FFL.