RE: tornado questions
The tires determine the steering torque required, the stockers only need a 3kg servo - a 10kg servo should turn monster truck tires with ease. The throttle/brake servo around 6kg would be good for squeezing the brakes with monster tires - if you're using buggy tires then a 3kg servo is plenty.
I've got a Savage spur in my tornado but it's not a direct swap. A couple others on here have put HPI gear in their cars, most require a bit of modding and crossed fingers to get to fit. But there are plenty of metal gear options for the Savage trucks so it's possible.
Do you have the 2-speed or the single-speed?