when you slow down the eng is making even less heat but you have way way less air flow/less water flow & less timing. As said you need more timing (I run 19 on a bone stock 85 318) & what is your pulley ratio (I'm assuming at least 1-1 if not overdriven, even better). what is your in gear idle speed (which is what it is seeing at the stoplight). Idling, how much suction do you feel on your hand in front of the rad? (some C bodies it felt like a tornado in there). for a shroud app, the fan 1" away from the rad seems a bit close to the rad (how wide is the shroud?). FOR SURE get a 180 stat in there, You want it to get out of the cold extreme wear zone FAST & a 160-180 stat has NO bearing on the running hot temp on the other end of the scale (210-220). Short version: you may just need more rad. Champions are pretty good nowadays & cheap, if you dont mind the square non rounded top tank. Holler how it turns out/what fixes it.