TC3/4's are a little difficult to get the FDR down lower for brushless, since the internal ratio is 2.5. You have to go with a pretty small spur. In 48 pitch gearing, I would suggest this spur since it is fairly easy to find and would be a good place to start:
In 64 pitch, something in the 82-86 tooth range would probably be a good target as well. We run 4.2 in our local VTA since we run 21.5's. So, I run a 84/50 spur/pinion. Like this:
Note: You will likely have to grind the chassis supports under the pinion gear to allow the large 64p pinions enough clearance.