Servo Vs Stepper for a high accuracy project
I'd strongly suggest you do not use either RC servos or stepper motors. 

Buy a Throttle servo designed for a car (many are drive by wire these days), or a cruise control servo.
These automatically lose drive if anything goes wrong, or when you stop signaling them.
There are plenty on Ebay or Amazon, just search for 'Cruise control servo' 

Think of the safety requirements, you cannot afford for any fault to keep the throttle at a wrong setting.