The radiator is not spposed to be filled to the top. There should be a gap of 1" between the top of the water and the filler neck. This is to allow for heat expansion. When you stop after a run,the heat from the exhaust mainfold is soaked into the engine( heat soak) and this heats the water. because there isn't any airflow over the radiator core,the water temp rises along with the pressure and the excess is spat out the over flow.
Your best bet is to make sure the engine doesn't go over 175 F when driving.
The top speed of your 250 should be 120+ mph. There is something wrong if it is struggling at 55MPH. Is it a C or CE ?