3rd party would be easiest
And even factory are self contained, only real tie in to trucks system is brake lights and clutch switch.
Factory steering column harness.
Cruise has 4 parts
1. Speed sensor, 1985 won't have a VSS, electronic speed sensor, so you need to add one, which is easier than it sounds.
A speed sensor just makes pulses when truck is moving, so most after market cruise setups just have you put 2 or 4 magnets on the drive shaft and mount sensor near them, so when drive shaft turns sensor generates a pulse for each magnet passing by it.
2. Actuator/servo, this is what actually pulls on the throttle linkage, it is connected to the engines vacuum for pulling power but can be all electric.
This is mounted in the engine bay and it's cable is connected to throttle linkage, carb or fuel injected doesn't matter.
3. The amplifier/brains, this can be inside the servo or be a separate unit, it "counts the pulses" from the sensor and adjusts servo, when engaged, to maintain the drivers chosen pulse count
4. The controller, located in the cab, often on steering wheel(clockspring change) or turn signal stalk for 3rd party, can be dash mounted as well on some.
This turns cruise on/off and also sets speed.
There will also be a tie in to Brake pedal switch, for shut off, and clutch pedal switch as well if manual trans.
No cruise control knows what "speed" is, it just see pulses from a speed sensor.
If driver hits "set" button when there are 100 pulses a minute then Amplifier will pull throttle open using the servo until it sees 101 pulses a minute then close it a bit for 100, if it goes to 99 then it pulls it a bit more, if it goes to 101 it close it a bit, and so on, that could be 40MPH or 80MPH, cruise has no idea nor does it matter, all it knows are pulse counts.
If driver hits "increase" it changes pulse count in memory to 101, Decrease goes to 99, very very simple setup
Coast turns servo off, Resume brings it back to 100, last pulse count in memory
If brake lights or clutch pedal are used then it turns off servo
The older vehicle Ford cruise controls, pre-VSS, would have the transmission speedometer cable run to the servo in the engine bay, and then a second shorter speedo cable run from servo to speedometer.
So servo was where pulses were generated.
You might be able to cobble together a factory system but most of these parts are long gone and may not work well even if you found them.