1) The gamemodel isnt what makes a truck solid, its the invisible brushes with the model.
2) Solid on those brushes is set by a spawnflag. Look in radient to find which one.
3) Find a map that has a truck similar to the one you want (ie, same number of barriers). Then copy the entities to your map/mapscript.
Another point, you'll find it extremely difficult to script the invisible brushes into existance, unless there's a entity truck of the same shape already on the map (ie, the brushwork already exists as a model in the bsp).