If, as you say, decelerating down a long hill triggers a full rich
condition, bingo, you have a problem in your idle/WOT switches or in the
circuitry. That's what I suspected some time ago when you posted about mpg
problems.

What I would do is, first, set the idle switch per Bentley.  Move it back
and forth several times with your light on it to see if  it retains the
setting. If it does not, replace the switch. Then, pull the ECU connector
and do the pin test, again following Bentley. When testing pin 4, first
actuate the throttle from idle to WOT, and then try to simulate engine
vibrations by pushing the switches and wires back and forth, messing with
them, to try to see if they're only working part of the time. That will test
the switches and circuit together. If it operates erratically, replace
switch and do what you need to fix the circuit.

What makes me suspect the idle switch (or circuit) is that you say it swings
to lean when exiting a freeway (normal) but doesn't do the same thing on
long downhills. Both conditions should be the same, although I seem to
remember mine swinging back from far left (lean) to near center after a
period of long deceleration. I also have an '84 with two switches and know
from experience how important they are. In any event, under no circumstances
should the ECU be saying "rich" during deceleration.

If in fact you are running rich on deceleration, that also accounts for at
least part of your poor mpg. The engine will run fine, it just burns way
more gas than it needs. The more you decelerate, the more important this
becomes.