69 Firebird conv. 350 overheating issue
 

I added Vintage air to my "69 convertible. It overheats after being on the highway then dropping down to slow traffic. Tried electric fan. A friend suggested a water pump pulley smaller (6") than the 8" stock pulley. Unable to find one anywhere. My 69 has the earlier 8 bolt water pump. Are their high performance mechanical pumps for this configuration? Does anyone know where I might find a 6" pulley (double groove). I saw a mention of someone "swapping" out the 8 bolt pump for the later 11 bolt with the back plate which the 8 bolt does not have. Is this really feasible?