I was thinking the cruise control on a powerstroke was done electronically by the computer. So you have the buttons and all the speed sensors, but the cummins is controlled by a throttle cable. So I'm sure it can be done, but you need to integrate a cruise servo with a cable into the mix. You may end up being stand alone as I'm thinking that getting the computer to control a servo is tough. Now you may be able to look at the cruise circuit from a 92-94 IDI truck or maybe gasser if they still use the servo an swap most of that system over. You could use your switces in the steering wheel and add the brake switch and other connections as needed along with the cruise servo and the cruise brain box.
Good luck. I'm sure instructions are somewhere on the net. As I'm sure many people have done this already.