63 Dodge Dart, now let me see;)

I know I'm gonna get it, but plumkrazee70 brought this to my attention regarding high volume water pumps. This site is about the only one advertising them. If the fan helps, would this be a good thing? We know my engine gets higher temps at idle, but fine at speeds. ok
got to make dindin, that's supper you know;)

1963-1987 Slant 6 hi flow water pump for the 170 198 225 cid blocks they make a high flow water pump for the slant? If so, get one along with a good high flow thermostat (degree of your choice) that should fix it right up.
My thought is, as you're driving the temperatures are okay. Then when you slow down, the water pump isn't spinning as fast. Temps increase.

Also check your pulley ratios. You want the wp smaller than the crank.