These trucks only run mid 60's full tilt. Just use a brick.
It looks to be easily done, though it needs electrical pulses from a speedo pulse generator as it's input, which our trucks do not have. You would have to adapt an automotive pulse generator into our speedo cable as a pass through, or have your dash speedo inop as it would go right to the pulse generator. Speedometer shops may have a splitter to do this, not sure if our cable connections are some type of standard, or unique. M920 seems to have had one, ask him how this portion worked (way cool train by the way).
The website and product looks to be built in a guys basement. Maybe ok functionality, but I'd be at least a little concerned about safety. OEM products are very hardened against EMC/EMI/RF interference, voltage spikes, reverse voltage, water intrusion, temperature, double jump start voltage, etc.... One unexpected electrical spike/pulse, even from a shorted out traffic light overhead, could trigger this thing to pull on the cable unexpectedly. I'd maybe use it with a big red mushroom stop style button on the dash that cuts power to it, and as long as the actuator is spring return and not worm gear driven (not self-holding). You are putting this thing on the throttle of a 21,000lb truck. The OEM cruise controls have many safeties built into them, such as auto-cancellation if vehicle speed increases/decreases faster than a pre-set threshold, etc....
See if somebody makes a similar actuator which uses self contained GPS, then you wouldn't need to splice in a pulse generator into your speedo cable.