I assume you have resolved your overheating issue by now. However, in case you still have overheating issues when idling in traffic on a hot day, I will explain an identical situation that I finally resolved. I performed a leakdown test on each cylinder and when I put air into cylinder 6, I heard gurgling in the radiator. I suspected a blown head gasket but when I removed the head the gasket was fine. I put the head on my workbench and using a magnafying glass found a small crack about an inch long that started from the exhaust valve seat. This overheating issue had gone on for about a year and I never got water in the oil and never had a radiator leak. As long as I was not stuck in traffic on a hot day, the truck had normal temperature.I relaced the head and the problem is fixed. My engine is a 283 V8. Hope this helps.
