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