I've found it way easier to retrofit a '70s cruise control.
You need the cruise control unit, the throttle linkage add on, the cable mount, the cruise brake light switch, the cruise control turn signal wand, wiring and the 2 piece spedo cable. I don't know when they went to the larger speedometer threads, I know my '66 doesn't have the large threads, and my '69 Roadrunner did (the car I added it to decades ago)
My $.05 (since they don't make pennys any more)