Pro chassis comes with a 16/42 gearing as well. I busted my 42 spur and put on a backup 46 spur with 18 pinion and I ran out of space on mount slider. Had to mill out a few mm to squeeze on the 18. I'm planning to run 41 or 42 spur with 18 pinion, which should work.
With stock 42 spur, I could not install a 14 pinion. So theoretically you should be able to use 16+ pinions. The RTR motor mount is different than the Pro but should fit same gearing.