I wouldn't be affraid of trying a lower kv option for 6s lipo use, something like 1600-1800kv would work quite nicely.
The castle 1717 would be great for that size of truck, it is 50mm in diameter but would be worth a try as it is currently only $70 ( special clearance price ):
http://www.castlecreations.com/produ...le_motors.html