I would consider the water pump a cheap test. Replace it. They corrode and clearance inside the housing increases between impeller and the casting.
Bypass fitting correct size?
How is the flow at heater core?
Heater fan on high, heat on hot. engine at operating temp, both heater hoses should be almost the same temperature.
I had a heater core problem and clamped hose down.it created an overheating condition. Food for thought.
