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This is just my opinion, but I think it would be enough of a pain in the neck (or other body part) that I wouldn't try it. Installing the components under the hood is not a problem assuming you can locate a complete set of components from another car. You need the control module, which mounts on the driver's side inner fender, and the vacuum servo, which mounts behind the carburetor. The only thing you will have to do with the speedometer is change the speedometer cabling. Instead of one cable that runs from the transmission to the speedometer head, there are two cables. One runs from the transmission to the control module and the other from the module to the speedometer head. So you will need to get behind the speedometer, or at least be able to reach up there and disconnect and reconnect a cable. The cables you need are probably available at Rockauto and other places. I needed to replace both cables on my '78 Toro, and Rockauto had both. Replacing them cured the wavering speedometer needle problem and eliminated a speed-related squeaking noise from behind the dash.
You will have to change the brake light switch on the brake pedal. It's easy enough to get at this, but you'll have to run more wiring and a vacuum line. The standard brake light switch has one switch, normally open, that closes when you step on the pedal and turns on the brake lights. For cars equipped with cruise control, there is a second switch in the brake light switch that is normally closed and opens when the pedal is pressed. This breaks the electrical connection to the control module and turns off the cruise control. There is a second, redundant safety feature as well in the brake light switch in that there is also a vacuum connection. You'll need to run a vacuum line from the line that connects to the vacuum servo to the brake light switch. When you step on the pedal, the switch breaks the vacuum to the servo and stops the speed control.
But none of the above is the real problem. The real issue is that you'll have to locate a replacement turn-signal stalk that has the cruise control controls on it, and you'll have to run wiring from this switch through the steering column. So you'll have to take the column apart to replace the turn signal stalk and run the wiring. You'll also have to locate a correct dashboard switch for the cruise control, and install and run wires to that. If you attempt this, you will definitely want the chassis service manual for your car. You want to look closely at the section on the control and see everything that's involved. You'll be running new wiring, new vacuum lines, and new speedometer cables, so you'll be spending lots of time pushing things through the firewall and getting at them down under the dash.
In short, you'll have to locate a complete assembly and wiring from a parts car, and you'll have to significantly disassemble the dash and steering column to install the components in addition to the additional brake pedal switch wiring and vacuum connection needed.
Back in the day they made aftermarket cruise controls. I know you said you don't want one, but hear me out. A very popular one was made by Dana Corporation. It worked by gluing magnets to the drive shaft and mounting a sensor near the magnets. Speed was sensed by the rotation speed of the magnets. You still had to put a servo under the hood, but there was no fiddling with speedometer cables, and there was no putting a switch in the dash or changing out the turn signal stalk. Rather, the controller for the cruise control mounted on top of the turn signal stalk, and you ran the wiring as inconspicuously as possible down the underside of the steering column. I installed two of these back in the day, one in a 1975 Chevy Nova and the other in a '77 Chevy Caprice. Both worked very well and actually gave more features than the factory cruise system because the aftermarket system had "resume" and "accelerate" controls on the controller, something that factory cruise controls, at least GM, wouldn't offer until a number of years later.
The controller that mounted on the turn signal stalk looked like this.
My point is that putting one of these on your car, if you could find one, would be about as period-correct as having a factory system. It would have been very correct for that era to see one of these installed on a car, and you'd save yourself lots of headaches in the installation process.
I agree w/ Jaunty. I put 2 of these on old cars and they were great. One a 65 Dodge Coronet and the other a 70 Vista Cruiser. I still have 2 of 'em in boxes, I guess I will sell them as I probably will never use them. If you are interested PM me. Or you may be able to find them still on Ebay. They come up from time to time.
There are a couple of aftermarket options. Vacuum operated or electric servos. Both will operate correctly and give reliable results. They can be speed controlled by the above mentioned magnets on the driveline or by engine speed from the tach/coil connection. I have installed dozens of these type of cruise controls and they can be made to look very inconspicuous. The servo and all other connections can be installed under the dash so the only thing visible is the throttle cable. Rostra still makes a replacement stalk with the full assortment of controls but you will have to remove the steering wheel to install it similar to a new turn signal switch. The connection to the carb is hidden by the air cleaner hat and everything else is inside the dash. If you are not WOWed by the stalk control, you can use a dash mounted control
If you are working on a 100% accurate restoration, best of luck to you because you will be working with all used parts.
Another option would be a newer NOS cruise like the one listed below. It is made for a late 80s GM vehicle judging by the turn signal stalk included, but could probably be adapted to a stalk that fits your car. This would probably still require magnets and pick-up on the driveline to simulate the PPM (pulses per mile) that a newer vehicle would have. It would require several manuals to reconcile all of the wiring. https://www.ebay.com/itm/NOS-OEM-GM-...wAAOSwubdeJir5
i have installed 3 or these systems (one on my 69, another on my dads 81 GMC, a third on a friends Malibu.) All have worked flawlessly, and are fairly easy to install. They made a speed sensor that goes inline with the speedo cable. However, if your car has a lockup torque converter, you may be able to use the factory speed sensor. I’m not familiar with the computer controls of the older G body cars. If you don’t want the hassle of a cable driven speed sensor you may be able to adapt the speed sensor from a later model G body speedo housing.
