I had a very similar problem when I first started driving my 32 roadster with a .060" over 350. Let's just say that I tried everything I could think of without any success. I now have driven it 8,000 miles with a unvented solid three piece hood, and hardly ever turn on the electric fan unless stuck in bumper to bumper traffic.

My cure was that I finally "discovered" on my pretty "no name" imported polished aluminum water pump that the impeller (I took the back of the pump cover off) was not even close to the pump casting, effectively pumping no flow of coolant. I bought a FlowCooler pump and never had another cooling problem. The fix is not always easy to see. Good luck.