[RC-Monster Mike]

4s Lipo would be my choice. I use the Kokam 3200 cells. If you only race for 5 minutes, these work quite well. I don't use any type of voltage cutoff device, but I only run for 5 minutes also. These are available at Tower hobbies, along with the astroflight lithium charger. They also make packs with a cutoff built in, along with the ability to quick charge. For longer runtime and safe running, a pair of the 6400 mah 2s scorpion packs in series would be pretty sweet. This is slightly heavier than 12 cells, but with 6400 mah of power and 120 continuous amp draw capabilities, the performace would be very nice(better than 14 nimh cells).
Sweet truck, by the way! I have a similar truck, but I use the towerless vbs and a custom rear skid to use the extension on the chassis(no wing on my truck, either).

Mike