Its a shame the TrueRC pack didn't fit. If it did, it would make my future conversion of the RC8 much easier...
jnev measure your pack (you have a 4s2p 10c 8amp pack don't you?). It's 50mm wide, but I think the 4s packs are about 45-47mm tall, so if you flip it on it's side it should fit well. Just make sure there's enough room under the body for a 50 mm tall battery.