What is the leading edge to firewall length, and what is the leading edge to spinner space? If too short, balance can become a problem. I would find a friend with a motor and battery to approximate balance, or figure a 7 oz motor in the motor space (1/2 lb wt to allow for prop/spinner) and about 12 oz of battery + add 2 - 3 oz for electronics in the battery compartment (1 lb weight centered in the battery compartment).
The HobbyKing info approximates various battery lengths and configurations.
Tim, wood block "standoffs" are easy and can be glued to the firewall. Last I knew, Fry's had standoffs in electronics so those are easily available. I didn't need standoffs, so just have a standard X to the firewall used as a rear mount. Not photo worthy, but, if needed..