The FP motor will fit in the 42mm gearboxes. you either have to pull the pinion gear off the motor that comes with the gearbox or buy
another one just get the right one for whichever box you get. The shafts are also a hair long so you will need to add some small spaces when mounting or cut down the shaft. One idea we were looking at was getting 2 of the 27:1 gearboxes and then using both FP's to dual drive a worm gear for our arm, although almost every member of the team hates the FP motors, which is not cool since a good 3/4 of them are freshmen and have never seen them in action.