Cross-Country Road Trip in a '68 Barracuda

Well, the car just boiled over for the first time just outside Craters of the Moon National Park... it's only about 73 degrees out and we were going up a mild incline, so we're gonna have to reevaluate the plan here. The thermostat is opening and the water pump is moving liquid, so I'm thinking I'm going to stop at a parts store in Idaho Falls and pull the thermostat to see if that helps.

For reference, it's an almost stock 360 with the factory 22" light-duty V8 radiator, old flex-a-lite fan, no fan shroud, and no radiator support seal.

I may also try to rig up a radiator sprayer system using the washer fluid reservoir. Any other ideas that might help?

...and no, I don't have a functional coolant gauge...