As shown in your pix the usual location for the battery pack is right behind tank where there's loads of room. Generally balance comes out quite well with it there. What size and type batteries? Generally 800- 1100 Ma. 4.8 or 6.0 v. ni cds are suitable. Since you appear from what you have said to be a bit nose heavy I would as said previously test fly it. Nose heavy is not a big problem as tail heavy can be. Of course the manner in which you check for balance could be questionable but will assume you know what your doing there. Off hand I would expect that if you were to move the bat. back behind servo tray you would definitly become tail heavy,N.G. By the way wrap bat. pack in foam I often use that which is used as underlayment of carpet, about 1/2 "or so thick.