Dodge Repair: Dodge Dakota blowing coolant out of radiator, head gasket leak, serp belt

Question
I have a 1994 dakota and it has a 318 engine.It is over heating by blowing the coolant out the radiator over-flow. The engine has 145000mi. It has a new radiator,waterpump,and theromastat.It seems to only do this if your crusing at hiway speeds.Is their anything common about this problem with the 318? What would be your guess?        Thank You for your advice!

Answer
Does the truck have a temp gauge?
If so, does the gauge indicate that the engine is actually over heating?
Does the heater blow hot air?

here are a few things to check


1) The serp belt is installed wrong, causing the waterpump
  to spin the wrong way.
2) The t-stat is installed wrong

3) head gasket leak

4) cooling system blockage