Have a 72 Dart 5.2 52,230 miles and it started overheating and what I found was a previous owner had put stop leak in engine. It built up in the corner of the back driver side cooling water jacket in the block. Found this when I removed the heads to replace head gaskets (they were not bad, just covering bases). The stop leak was intended to stop a leak at rear water soft plug in rear of block. Don't know your history on your car, but might be a possibility?
