I am now of opinion the '79 is a mechanical linkage to throttle??? How unlucky to have different controls on each car!!! Just came across this:
https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/615/30-300.pdf
On the '79 it has Engine Model: 617.912 and Trans Model: 722.118 and I find the 1980 has same engine and trans. Why in hell did they switch the controls???
So for the 1979 should I be looking at Section C on page 10 of 17 in the PDF????? However I find the pics and descriptions impossible to figure out.
But mechanical linkages are unlikely to change so what else could the problem be???
Car drives and shifts beautifully on level terrain. But get it on a hill and it is a dog.