It's tough to make a solid diagnosis over the internet, but it does look like reduced airflow into the radiator and an engine being worked very hard led to overheating and the radiator cap released the pressure. Probably does not have a coolant recovery bottle so it just spit out of the top of the radiator. Should be a little nipple in the neck right below the radiator cap. There might be a small hose attached to it. Might also be as simple as a bad cap not holding pressure. Check for a rubber gasket on the bottom and see if it has a springy feel when you push on it.
Your car or truck moving along the highway generates plenty of airflow through the radiator. Your cat working it's heart out at maybe 2-3 MPH has no airflow. If the fan can't pull enough air it will get hot.
You definitely need to learn how your cat works and be able to repair minor problems. Carry a tool kit, extra coolant, oil, maybe a belt and some hoses. Some electrical bits. Easy stuff to fix if it breaks. The chances of getting a mechanic out to fix it are about slim to none. You guys look pretty well dressed for the outdoors. Carry extra food, water, clothes and sleeping bags in case a breakdown turns into an overnight stay in the back of your icebox/1500.
Radiator might have a drain petcock on the bottom, or it might not. Or it might twist itself off the radiator. Ask me how I know that.
Do you have the Ford or AMC engine? Some engines will have a threaded plug in the block to drain the coolant. The Ford 6 has it under the exhaust manifold.