Anybody know of the FDE modifications to make the regular iFDG A318 into the ACJ variation?

The only real difference is the fuel tanks. I would obtain the fuel specs of the real ACJ and modify the aircraft.cfg accordingly. This will require a seperate aircraft folder in your library from the regular A318.Also, you could obtain the IFDG A319 and A319CJ and compare those aircraft.cfg's. That will give you a good idea.