The spark plugs will give you an idea if the engine is running lean and making it run hot, however if it was runninglean it should heat up all the time, just not w/the extra load of the A/C.
Do you have the stock fan shroud still on?
I wasn't sure if there was enough room to squeeze a small A/C pusher fan in there or not. I installed one on a buddies '83 for the transmission cooler(he had a Vortec swapped in it), but he had the grill off, and I have never bothered to look how he has the grill attached. There might be enough room to squeeze a Toyota Supra fan in there, I will take a measurement of the one on my '87, it isn't very deep.
You could try 50/50 mixture using distilled water over tap, though Ia m not sure if the minerals found in most tap water would be enough to cause the over heating issue.
IIRC, it only over heats when sitting in traffic w/the A/C on, not when cruising down the highway, right?
Are you still running a R12 system, or has it been converted to R134?
