Slant 6 Overheating

Ok, folks: 1971 Dart, 225 /6, non-A/C. Never had an overheating problem; in fact, in the wintertime I complain my heater doesn't blow hot enough air (new core, so no problem there...).

Yesterday, on a 15 mile ride, I noticed after about 5 miles that my temperature gauge was up around 1/3-1/2. Not quite to the half-way mark (non-rally, stock dash & gauge). After about 10 miles (cruising at 65-70 on the interstate), I started smelling coolant. After 15 miles and when I pulled into my driveway, I had pretty much lost all my coolant through the puke bucket, which doesn't have a cap on it. Coolant all over the engine compartment. Big mess.

This car has NEVER overheated before. Why this all of a sudden? So, this morning, I filled up the radiator (with plain water), fired her up and let it sit until the thermostat had opening and I could see that things were flowing, put the radiator cap on, and took her for a spin. Everything was fine, temperature gauge at the 1/8-1/4 mark as per usual, and after about 5 miles the gauge crept up to 1/3-1/2 mark, and I headed back to the house, where I found that I had again lost my coolant through the puke bucket. When I stopped the car I could hear a humming noise (engine still running) and when I popped my hood the puke bucket was boiling over and the radiator cap was doing the humming and vibrating, as coolant was boiling out of the radiator and into the puke can.

Ideas? Here are mine: Water pump, or thermostat, or radiator cap. But I don't quite know how to evaluate the problem and would rather not replace the water pump. I guess I could start simple--do the radiator cap first and see what happens, then if needed the thermostat, and lastly if necessary the water pump.

But if anyone sees a clue here that I am missing, I sure would appreciate your advice ! ! ! Thanks.