I would be looking for one of those nice 7 blade fans that Pontiac used in their later cars. My 69 T/A has one from the factory. I would say the temp your running at 75 is still not that good ( it would be a Stanley Steamer here in Phoenix) I agee with George, it sounds like an air flow issue. Are there any seals between the rad and core support? Fan shroud? What's the pulley size on the water pump? Some high output cars had bigger pulleys on the pump to slow it down for more power and putting a smaller pulley on may just give you that extra flow that you need at idle. Have Fun
