Dragon,
We had serious problems with the March pulleys on the FE motors. They are undersized. What happens is the water in the radiator may get cooler because it is in there longer, but the water in the heads boils and everything goes to hell pretty quick as you have now fallen off the cliff and no amount of cool radiator water will save you. At idle there is no air flow, by definition, so you have to do something else.
One day while working on a customer's car that was particularly troublesome I noticed the original pulleys were very close to the same size, or, a 1:1 ratio. The water pump was actually running a little slower than the harmonic balancer pulley on an original car. So, I changed the pulleys on the customer's car and all the overheating problems went out the window!
So, we made a new set of pulleys that actually over drives the water pump. All problems gone (in that department, at least).
That said, there has been a lot of good info posted here. But, there is one more thing we found. We put bypassing fuel pressure regulators in our cars--complete with return lines to the gas tank. That way the fuel can't get hot sitting on the intake manifold. Vapor lock and hot engines are all exacerbated by hot fuel. These are all small things, but they all add up to an overheating motor so you have to pay attention to all the details.
David
