I need about 6oz in the tail of a GP Christen Eagle to balance it out. Putting the battery plus another oz together in the tail will balance it properly. Any words of wisdom or cautions about putting the battery in the rear of the plane?

Make sure you have a heavy duty switch harness and heavy duty extension cable to run the long wire. Heavier wire will provide lower resistance, and that means less voltage drop betweent he battery and RX. Also... less heating of the wire under flight loads.

There is a possiblility of an RF interference if the battery cable parallels the RX antenna. If possible I would route the antenna appx 30 deg off angle from the battery wire.

No really special attention is needed.
Make sure that it is really secured in the tail

You might want to provide some type of hatch to get access to the battery so you can replace it later.

Tom