Does the cruise control tie into the ECU or the throttle?
Yes.
The unit splices into six wires on the main wiring harness right before it goes through the firewall, and has a linkage that connects to the throttle cable linkage. I'm not sure how it handles the dual-throttle-cable of the Gen I (do they offer a version for the Gen I??), perhaps they have a version with two cables coming out that connect to each of the dual Gen I throttle cables?
You will need to spend some amount of time matching the wires based on which position on the main connector, because, as I recall, there were a couple of instances where the same wire color code is used more than once in that harness, so you have to identify which one is which based on the position on the firewall connector.
Otherwise, not a real hard job. Budget an afternoon, get a comfortable seat (or sit on the ground) next to the wiring harness, and be ready to be on your head in the driver's side footwell in order to place the receiver (stick to the left side wall of the foot box). Definitely solder all wiring splices.