As much torque as you can afford! 1/8 nitro obviously put a lot more strain on the whole steering, even electric converted there is more mass to crash. Personally i run the HiTec 5955TG - it's badged as a Robot Servo and has 330oz of Torque with titanium gears. It's not quite as fast as some others (By a shadow) but it's plenty fast enough. I got my first one second hand about 6 months ago and it has been rock solid through some serious track time and racing.
http://www.amainhobbies.com/product_...oducts_id/4478
BTW, blue loktite the nut on the servo saver spring on the RC8 when you have found the right tension. It likes to loosen.
Hope this helps!