Take the radiator overflow tube and run it into a container. I have used a tall quart jug. The container that Hypoid oil came in. I cut off the tip enough to get the hose to the bottom of the container. I cut the bottom of the hose at an angle so it doesn't suck to the bottom of the jug. Poke a nail hole near the top of the jug somewhere so it can breath. Fill it and the radiator to the top and drive it. After a few times it may overflow the container but will set itself to it's own level.
Also, when you shut the truck off after a run, the heat isn't circulating anymore and the anitfreeze temp goes up and boils.
Try letting the engine idle for a few minutes when you stop. That will help cool the antifreeze a bit and may stop it from over flowing.
I remember replacing a radiator once. It was plugged (on the outside) with mud dirt and bugs. I always had a problem flooring it to pass someone and blowing the hose off.
You might want to have the radiator boiled out and cleaned if nothing else works.
