Since it sounds like you already have too much weight in the plane, why not put in a UBEC instead of an additional pack?

Try to move the battery back to adjust the C.G. even if it means having both packs in different locations.

You can put the recever anywhere you want, but remember to try to keep the antenna(s) AWAY from any internal wiring or metal.

Since you over did it with the motor, also consider putting in a smaller prop than what the motor calls for.

This will make the plane fly far longer, put a lower strain on the battery, while still giving you good performance.