I checked a few months ago and you can still get an aftermarket Rosetta Cruise control. They also have an extra cost switch that will allow the cruise to be added to a manual trans car.

Many years ago, I added an aftermarket cruise to my 70 Road Runner that had a 4 speed. There was a bead chain and a switch that connected between the clutch pedal and the brake pedal that was under a load when both pedals were up, as soon as you stepped on either the clutch or the brake, the chain went slack and the cruise disconnected the same way it would if you stepped on the brake with an auto trans. When both pedals were back up, and you were in 4th, you hit resume and the cruse was back working. It really worked well. I believe the Rosetta Cruise is an updated copy of the same one I had on my RR. Gene