If your engine is a 5.7 there is one of 2 problems you may be having. Your radiator may be clogged not allowing enough water to circulate fast enough through the engine, or you have the head gaskets on the wrong side, shutting down the water flow through the block. A lot of this just depends on how hot is is really running. Have you tryed a manual temp gauge on it to be sure it is not a gauge problem? Did this just happening, what caused the engine to have to be rebuilt. Also have you checked your fan clutch. Does it get hot idling or all the time?