Well, when under load, like pulling up a hill, I had been wanting to replace the radiator, where there are no heavy duty ones for the 68-74 any more all are standard, so the ac upgraded ones are no longer available, but I believe I narrowed it down to the hose collapsing, it's soft and has no spring in it, and I'm having trouble finding one with the spring. My van is a 73, when I bought it, someone had inserted a pipe in the water neck and punched the thermostat in, to disable it without having to remove it, pretty sure this was to stop it from overheating, not sure why they'd not just replace the lower hose, I installed an expansion tank to recover the coolant so it can pull it back but it empties that too. It doesn't do this often, just when under load, there is no fan shroud, and there are none available for my van either, have thought about an electric fan but it's not doing it in traffic, it's heating up while moving, and if 55 MPH isn't getting enough air through, the fan isn't gonna help, yet when going down hill, even when down shifted using the engine to brake it, it cools off fast.
Am I correct, is it the lower hose collapsing, or is the radiator not enough to keep it cool?

 

Should be pretty easy to test this out. Warm up the van and wait for the thermostat to open. Then have an assistant bring the RPMs up while you visually check the hose.

I haven't heard of this being a common thing, but I"ve never worked on something this old before. Would stand to reason that the rubber deteriorated to the point where it couldn't hold its shape.

 

Only does it pulling a hill so far that I've noticed, I went ahead and bought a new hose, and antifreeze, it was due for a coolant change.

 

You could re-core your Rad as well ...Just in case ...

 

No recore, I want heavy duty, mine is not, and all the shops around here have closed down, dad and I were talking about that, I'm pretty sure it's the hose collapsing because it's so soft, may have to get an aluminum radiator made.