I know it's been said, but if the tubes are closer together than normal, or the fin count (# of fins per inch) is higher it is harder to get the air to flow through the core. If it heats up only at idle I would expect the fan isn't drawing enough air at that speed (as from what you describe the engine as I can't fathom it being not enough water flow). I would recommend using the shroud you are building, as the smaller pulley will increase flow all the time (water and air). It (the pulley) would likely cure the problem but I would worry about cavitation at higher speeds and wear on the pump itself (but that's me).
If you are concerned the manifold heat is bypassing too much water around the radiator, could you put the water back in to the radiator (top tank or upper hose)? I expect you have it plumbed up so it returns to the suction side of the pump. I know that would reduce the flow (less pressure difference)when warm, but if it is still a problem, that could help. You should get more than enough water flow when the engine is cold (with the thermostat closed) to warm the manifold as quickly as it does now.
As for what kind of fan, I have used flex fans, steel clutch fans, electric fans,... I have had success with all. It seem that one of the car mags will run an article every 5 or 6 years on Clutch V/S Flex fans. Every time I have seen that the thermally clutched fan with the steel blades flows the most air with the least HP draw. I guess that's why all the OE's (domestic anyway) had something like it for their heavy duty applications (I'm sure they spent tons of money trying to get something cheaper to work). But whatever works.
