I'd probably use a big outrunner or dual KB45's, or 1 HV KB45.
Going hv with a kb45 sounds good. I have 2 xl kb45s and have been wondering what to use them in... I will say that the L can motors can draw some juice, but seem to stay reasonably cool in my gtp. I am sure 1 with 8-10s lipo would be plenty to move a heavy 6x6 truck!