Do you have a stock chassis to use? Would be much lighter tht way. You could take out the diff and tranny and put an outrunner where the diff would be. I bet you could get the truck under 6lb rtr with that setup on 4s.
You could try something like this:
http://www.hobbycity.com/hobbycity/s...less_Outrunner
Maybe have mtxmugen rewind it to ~180kv.