go here:
www.holmeshobbies.com
Use code "ESC" after selecting the Mamba Monster combo you want. that should bring the shipped price of the Mamba Monster combo to $250.
As for the conversion to use for an RC8, I would give the RCM conversion a serious look as it moves the battery pack weight over to be more centered. I have this for the RC8T, and its great. I also have the RCPD RC8 conversion which is great if you want to use long packs.
Main thing to keep in mind with dual lipo setup is you will want a charger that charges both packs together as one and balances them too, as one. the Hyperion duo does this, and no manufacturer currently will make a balancing Y connector so you could use a regular lipo charger.
good luck.