Jimmy,
You'll need to reduce the steering throw a bit, and chattering is normal with a spool in just about all the cars on the market (SD, TRF415...etc.) remember, spools is a very unforgiving type of transmission components (it does not compensate for the different radius of that the inside and outside tires are traveling when going through a turn). We ran the spool at the reedy race this year, and on the bench the car chatters when the steering was cranked all the way, but on the track we didn't actually notice any chattering. Reason being, most of the time on the track you rarely crank your steering wheel all the way. Spools works best on wide open tracks where you rarely turn the car through tight angle turns, and/or when the traction is low and the one-way is difficult to control (for example, large parking lot type races).
We've also tried Yokomo SD parts (SD-415S or SD-415SG) which reduce the ackerman a bit and relieve some of the chattering.
Steve Wang