Back in the 1974-era, when the national speed limit had become 55mph, there were some independent shops and Chrysler dealers that were doing "big business" in installing Chrysler and Perfect Circle cruise kits. I did my own looking around, too. The plan was to find something OEM-related for our '66 Newport Town Sedan 383 2bbl car.
As to the OEM-related items, it seemed that the speedo cables were the same as the new '74-era Chryslers. Same placement on the inner fender. Same placement of the items to the carb, too.
At the time, I was in college in Lubbock, TX at TTU. I naturally took the Chrysler to Fenner Tubbs C-P if it needed anything. I asked the service advisor about putting one of the factory accessory kits on the '66. He referred me to the tech that did those things. I told the tech what I was interested in. He confirmed that the factory kits for 1974 cars would fit. He also took out a pen and marked how much the steering column piece where the turn signal was would need to be filed out for clearance of the wiring of the new turn signal stalk. Past that, no other mods were needed, he said. Basically, the existing hole would need to be enlarged, double its front to back side, in the direction of the instrument panel.
I also went to a private shop which did the Perfect Circle kits. Which was Chrysler's supplier for the cruise kits back then. Same story from them, just enlarge the turn signal stalk rectangle, toward the instrument panel, and everything else just fit like it was designed for it.
Some aftermarket kits used magnets glued to the driveshaft to measure speed with, by way of a sensor that had to be mounted to the floor pan. No need to deal with speedo cables that way. With time, the "glue" would age and magnets left the tube.
On later versions, they would use a signal generator that screwed directly onto the speedometer cable adapter on the transmission tail shaft housing. Then the existing speedo cable was screwed onto the signal generator. Probably more accurate!
ARA had cruise kits (for everything) that looked like a re-packaged Chrysler-type unit.
On many aftermarket kits, the sequence of the slide on the turn signal stalk was backward from the OEM sequence, which had "OFF" at the left of the sequence and "RESUME" at the far right end of the sequence. Might take some getting used to.
You will ALSO need the longest carb throttle linkage stud. This is available from Holley and Edelbrock, but would also be on every salvage yard Mopar that came with a factory cruise control.
Now . . . you just need to find an unmolested Formal Chrysler in the salvage yard and get all of the under-dash and under-hood items off of it. If you are lucky, the servo will hold vacuum! Otherwise, might need to find a reman servo somewhere.
Never did get that cruise kit installed, though.
The results of my investigations, back then. Enjoy!
CBODY67