... but I don't know how to interchange the gears on a truck.
"Don't" unless it's to a matching gear replacement. "Tolerances" change with different gear sizes! A given power truck is manufactured with the needed gear tolerances for what it came with from the factory...it's not so easy to just change gearing in any power truck unless it's the same thing as it came with...or I would've changed mine eons ago for slower running!
BTW- what is the rear trucks axle spacing- center to center...just curious?! Scale feet or real time inches- either measurement is fine.