An Avistar will almost always wind up very nose heavy with anything other than an LA motor in it. The battery is best put right behind the radio compartment, underneath the guides for the control rods. It's a very tight fit back there, but you could always make a hatch and install it that way. I would also suggest, if you don't have one already, an 1100 mAh Ni-Cad, the extra weight should make it balance perfectly.

Andy