The gears and shaft are easily capable of holding the torque required to remove the nuts without issue. The engine is turning out more torque than that and with the gear reduction it's much greater. Not to mention the torque spike those shafts and gears see if you sidestep the clutch.
The problem is getting the driven gear off the output shaft. It can be pressed on really tight and there is hardly any space behind it to get a puller in. You can't get a bearing splitter in either because the shift shafts and the input shaft block you. You don't really want to heat the gear up if you plan on re-using it, so that's not a good option.
This is the step I'm stuck on now. I have a grenaded WRX transmission that the only good gears left are 5th and I want to take them off with the transmission case together so I have a proof of concept to put it into the Legacy Outback trans in car. I've already broken one puller.