Something like this would probably be ideal for a half-track conversion. Basically a modified MGT chassis with a funky centre diff and six wheel drive. I actually saw one of these racing close-up (i was marshalling) and it was incredible. Very stable and fast with an OS .30. The only bad thing about it was that it didn't handle the jumps too well....
http://www.beatyourtruck.com/prodrev...ckstop6x6.html