That's pretty much what I have now from Griffin. I'm using a Milodon water pump, and a 6in crank pulley & 6.50 in water pump pulley. Ron Davis radiator in Glendale Az., suggests using a 7in crank pulley & a 5in water pump pulley with their (of course!) radiator.
Yup, it ain't cheap but I'm not sure if the problem isn't simply the pulleys