Just a few thoughts.. IF you go to any junkyard and look under the hood of a gm vehicle with cruise.. look for a black rectangle box on the firewall with a throttle cable attached to it.. Approx.. 4"X6"X2" Caprice,pickup truck,etc anywhere from 91's to 2004 probably has them.. when you get one, I can send you the schematics for the vehicle, but they are mostly the same anyways.. This will have all the hardware you want, including safety disconnect.. If your lazy, all you need is power,brake input,speed input and switch input to make it work, but you could still use an Arduino to run it if your looking to DIY.. This would give you a serious jump on the hardware.. I really don't think the stepper motor you describe is up to the task, there are usually 2 springs pulling on the throttle to close it and they have a bit of force..