Hi, Mark.  Going by memory from my 88 GL, the heater, etc. controls on the
dash are as follows:

Top lever (1) in the left position directs the flow of air to the floor, in
the right position it directs the flow of air to the floor and to the upper
dash top vents for defrosting.

Second lever (2) from the top is the heat setting - in the left most
position is the natural, unheated air temperature, and as you move this
lever to the right the heater core will start warming up the air flow.

Third lever (3) down is another directional switch - moving it from the
left to the right will start increasing the amount of airflow to the two
air ducts on the opposite ends of the dash, the duct to the left of the
instrument panel and the one to the right of  the glove box.

Bottom lever (4) is the fresh air control for the ducts that run along the
sides of the interior of the back of the van.  Far left is open, far right
is closed.

To answer your specific question for maximizing your AC comfort, I do the
following for the Dallas hot months (April - Oct):
-Fan switches off
-Close all the ducts in the dash and all the ones for the fresh air in back
along the sides
-Levers 1, 2, and 3 all the way to the left
-Lever 4 all the way to the right

But, as there are 4 levers with 2 positions each, and the fact that some 87
do or don't flow outside air into the dash ducts, I'm sure that you'll get
quite a bit of variation from other list members.  One of the coolest (no
pun intended) things I like to notice while running the air is that feeling
of rushing cold or hot air when you make sharp turns, especially down near
the floor.  Check it out sometime.

Karl
88 GL