I have the 1980 with cruise control that uses the can and cable and vacuum
to control the throttle.

I searched for anyone saying they had adapted the type on mid 80s cars onto this older system.
Did not find it.

If after market cruise controls can be adapted (I found a thread on that),
how feasible is it to install a cruise control from the 1983 style cars onto
this 1980 style ?

I am trying to install something that appears more reliable, maybe I am wrong.
Maybe the 1980 style is fine, as long as the parts are good.
Maybe the 1983 style is just as problematic.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

I'm sure it is doable. You would need the bracketry and linkages to mount the actuator to the engine and create a new wiring harness (which would be easier to do than make the other harness work). Would be best to find a junked car and get the wire connectors and all the hardware. The speed sensor and switch I'm sure are the same. I had looked into it in case I came across a car without cruise.

As far as the new style being more reliable, I doubt it is. I read threads on this site all the time with people having trouble with the newer style as well. It is usually the same problem for both systems: the amplifier. I have a 1980 with the vacuum system and, after I had the amp fixed, it works perfect. On another, I had to also change out the actuator. If you think about it, the vacuum system is what most cars had and still use today. In my opinion, it is not worth the effort unless you already have a complete newer setup that works and you can change with no cost.

If you need to repair your amplifier or actuator, check out

http://www.gdl-online.com/

They do good work and offer a warranty on their work. I have had two amplifiers repaired (different cars) with great success.