That gearing sounds a bit tall for a truck. Try dropping down to 12 or 14T on the pinion and add the fan.

16/46 was what I ran in a buggy on 4S. The tires on the truck are a lot bigger in diameter and weigh a lot more too, so you have to account for that.

Whatever fan you get, 3mm self tapping screws fit perfectly between the fins of the MM controllers.