Years ago I put an aftermarket unit on a Chevy Bel Air I had. It had magnets that were fastened to the drive shaft, and a sensor that mounted near them. There was a servo unit that went under the hood, with a cable that hooked on to the carburetor linkage.
I'm sure you'd have to look a bit to find such a unit today, but I'm sure there's one out there somewhere. It can be done.
