Quick question I have a 67 400 and want to know if I change out my 8" wp pulley to a 6.5 wp pulley and leave my crank pulley 7 1/4" will there be any gain in cooling? Cooling is good when driving and hwys 190 degrees it's just when idling in long line ups.

It sure will help. i have a 5 inch pulley on my water pump.

Couple things to consider.
With a 400 do you have the lower valance in place? Do you have the upper filler panels installed? These are 2 things that many people removed at sometime and never reinstalled. Very important to have these in place for proper cooling. Another item is the water pump, many times the water pump was replaced but installer didn’t pay attention and may have not installed a high vane pump like what was installed by the manufacturer. The high vane water pump helps to reduce cavitation. Cavitation causes high temps do to poor coolant flow.
Have u tried a high flo tstat? This helps considerably too. Also check if your radiator is 4 core if it’s a copper radiator and has sufficient flow.

I bought my 69 in 1970 and have had issues u described, plus severe pinging when starting out and going up hills, temps would get up to 215 or higher on the gauge at low speed, over the years I have tried about everything with no significant help. I have the. Lower valance, upper filler plates, high vane pump. Last year the radiator had white spots showing signs of leaking so I replaced the radiator with a 4 core copper from US Radiator, installed a high flo 180 degree tstat.
This totally illuminated all my issues.

Define high vane pump ?
they only make 2 water pumps, one with cast impeller and another with stamped steel impeller.
As long as the clearance to the divider plate is tight, then both work.
As far as the filler panels go....
Maybe maybe not
More important is what engine fan set up.

I do have both upper and lower filler panels, an aluminum radiator a stant 180 thermostat, water pump with cast iron impeller with proper clearance of the divider plate. I will try a hi-flow thermostat. Thanks for the info it's much appreciated

Forgot to mention I'm running a clutch 7 blade fan with factory shroud. Maybe I'm just being paranoid lol

Like Bigchief mentioned, the divider plate has to be a good fit. I took a small hammer to mine and it lowered the temp.