If you have everything hacked out of the truck, I doubt cruise will work. On some trucks the cruise is disabled if the check engine light is on.
You'll definitely need some sort of speed sensor on the output of the transmission.
You should be able to set it up as a pto system. You'll need DDDL on a laptop to go in and change the parameters in the engine computer, that should enable the pto option in the ecm. I don't know if you can use a variable resistor, but you should be able to use a regular accel/coast switch to bump up/down the speed. You might possibly have to have the clutch switch jumped to get it to work with this option as well.
What I would do would be to switch the governor setting in the ecm from limiting speed to variable speed, then I would use a c-clamp on the throttle pedal to adjust your speed.
That's assuming you have the throttle pedal out of the truck and hooked up. You'll have to do that anyways if your trying to use cruise control.
IIRC you can switch between governor options on a series 60(It's been a while since I've worked on one)