Man, overheating problems are a pain. My '67 starting overheating the night I took delivery of it! Went through all the logical steps: new t-stat, new water pump, new 5-core rad. Finally had the coolant tested for the presence of exhaust gases (flourocarbons??), and they were found, which meant, basically, that somehow exhaust gases were mingling with my coolant. So, pulled the heads and the gaskets looked fine. Sent the heads off to get hot-tanked, and no cracks were found. Finally discovered a crack in my block, which was letting exhaust gas into one of my water passages.
Don't mean to scare you, but you might want to get you coolant checked. If exhaust gases show up, you might have a blown head gasket, cracked head or cracked block.
Good luck!