I saw another post about a guy who was having some problems with his car after he installed a lower temp themostat without changing his PROM. I was looking at changing to a 170* thermostat, but what in the PROM must be changed to accept the lower temp? (1990 SD, $8D)

I don't have an exact answer for you (I'm new to this myself), but with the lower temp themostat the car may not go into closed loop properly.

There are multiple temp related values that are in the PROM. Check out Moates.net and download a copy of TunerPro, and you can see all of the values / tables that are in there. Craig can also hook you up with everything you need to make the changes.

Read the stickies at the top - a lot of help in there.

Good Luck.