Ford 302 rapid temperature rise at idle

[UKjohnnycobra]

Hi all, I'm looking for some advice on engine cooling.

I have a stock Ford 302 in my Cobra which I currently run with a 160F thermostat. The engine runs at around 180F when moving, but the minute I stop at traffic lights or in slow moving traffic, the temperature rises very quickly (less than a minute) to 200F or greater and the elecric fans can just about stop it going above 200F but cant get it back down to close to 180. When I get moving at speed again the temperature comes back down to 180F after a couple of minutes. The car is not boiling over but this is very annoying as it means that the fans are on any time I am stopped and for several minutes after I drive away
I have a new Griffin two row aluminium rad which is more than enough cooling. I have two 12 inch fans which are definitely blowing in the right direction. The themostat is new and drilled at 12 oclock. I have swapped out the stat and the problem persist with different temperature stats. The water pump is new. The problem existed with the old pump too. I have a bypass hose in place. There is no air in the system; it has been filled and burped multiple times an I can check the head fluid level through a fitting on the manifold. The block, heads and rad are completely filled. I use a 50/50 mix. I have flushed the system multiple times.

I am thinking that maybe the pulley ratio is wrong and the pump is not turning quickly enough at idle. I have a 6inch crank pulley and a 5.5 inch water pump pulley. Any thoughts on this? What are the correct pulley sizes/ratios?

Thanks in advance