There has got to be some problem that has not been addressed? You've made sure your gauges work correctly, and or used a Thermal temp sensor gun etc? You've made sure your timing is set right, and your not running too lean? And you've adjusted the devider plate to get it as close to the impellar of the water pump as possible? etc?

There has to be a mechanical cause that we are over looking?

But until we can figure it out, here are some pix from GTO Ken for that type of set up he tried:
http://home.att.net/~gtoken/birdwork10.html

I hope we can figure something out for you, and soon!

Good luck - Brett - 67 Drop Top 455!