Contrary to popular understanding, the fan does not suck air thru the rad.
The atmosphere on the outside of the car, pushes it's way thru, in response to the low-pressure on the engine-side of the rad, which the fan is attempting to create.
Stopped and idling is the hardest time for the cooling system.
Your prime concern is to reduce the pressure on the engine side of the fan, and to make sure the air coming thru the rad has some place to go.
The fan is the workhorse.
How it works is that the fan is trying desperately to create a low-pressure area inside the shroud, so that the atmosphere can push air thru the rad from the outside.
If ANY AIR from the engine-side, sneaks around the blade-tips or over the top of the rad, or thru the sides, that will reduce the air pressure on the rad from the atmospheric side.
And any air that sneaks in from the engine side, that air is already HOT, and expanding making the job even harder.
The point is, that the rad has to be "sealed" to the core support to prevent all that sneaking around.
After that, the OD of the fan has to be a tight fit in the shroud, with just enough room allowed for engine torque-over.
After that the fan has to be positioned front to back in the shroud in such a manner that the air coming thru has room to get "flung" off the blade tips radially.
After that you can talk about number of blades, and impeller vanes, and ratios etc.
Now, I just want to mention one more thing;
Your pulley ratios are from about 5% overdrive to about 5% underdrive, right? If your engine is equipped with a one to one ratio and idling at 700 in gear, the 5% increase is 35rpm right?
But if you currently are 5% underdriven and swapping to 5% overdriven, that would be a change of 10% right? Thus from 700 to 770 rpm, a change of 70 rpm.
But if you put the trans in neutral and the Rs jump up to 850........ that is 150 rpm and a jump of 21.4% from 700 . The point is this; just put the trans in neutral and see what happens! If the temp continues to rise, then a pulley ratio change will be a waste of money.
But check out what happens at the other end; At 6500 rpm, the difference of 10% is 650rpm from min to max . You better have a clutch on that fan else HotRod magazine says you will lose 600 horsepower, or some other slightly less-stupid ridiculously high number, lol. And, to that end; I highly recommend a HD Ford Thermostatic clutch from an early 2000s pick-up, on; an all-steel, 7-blade, high attack-angle Chrysler fan from a mid-70s A/C car. Mine runs at exactly 205 to 207*F all the time. Including at 550 rpm and 5* advance , chugging across the parking lot.
