I'm running a 7XL with a MM in a buggy that I just converted. I've only run it on 3S, so far. Still waiting for the 4S packs to come in. After having run the setup, I'm finding that maybe the 7XL is too low a wind for what I'm doing with the buggy. With a truck, you might want to consider an 8 or even 9XL. Something in the neighborhood of a 2000kv rating and run it on 4S Lipo.

Of course, I've never done this in a truck, but I can tell you that the 7XL is a pretty hot motor. General experience with brushless tells me that in a truck that you are likely going to bash with and abuse, you'll want a higher turn motor for cooler running. Believe me, 35 - 40mph in a truck is plenty fast too.