Does 80 use the "Dayna" system or is there an earlier type? I have the Dayna system on my 84. It works Ok, but it is not as accurate as computer newer types. Mine sets a little faster then I am actually going. There is an ajustment for that but I have never gotten it perfect.
The main problem is that it is driven by engine vacuum. A real problem for me because mine is on a turbo. When it goes up hill the turbo boost kills the cruise. It is probably OK on a NA car.
If you took note of where all the parts were off the parts car you should have no problems installing it. The throttle cable is intake manifold specific. There are different length cables depending on the intake and where the vacuum actuator is mounted. IF your car is identical to the donor you should have no problems.
Both Bentley and Haynes have wiring diagrams for this cruise. At least on the 81 to 85 speedos you can just add the pickup to your own. If your speedo is jumping because of a bad cable or speedo drive it would affect the cruise.