Hi Bigdaddy!
I haven't seen this thread for a while..
Well, i don't know about the hellfire, i haven't looked at it's exploded views, but with the HB lightning i did, the problem was that the centerdiff was off-center.. i made a 5 mm thick plate, that centered that little bugger right in the middle of the frame, enabling me to have the batteries on eacht side.
I dished the RCbox, you don't want to work around it.
I also replaced the steering servo, and i don't know if you are planning on using the mechanical brakes.
Because in general, pictures tell more than thousand words;
here are a few pics to give you an idea of how i managed the batteries..
I hope they can be of any help.