May as well add my 2c worth!
A common problem with re-built or engines that have laid idle for a while, can be loose scale in the water jackets.
If the new engine block was not caustic washed, or has sat for a while out of the vehicle, all the rust scale breaks away when the engine is first run and becomes lodged in the cores of your brand new radiator.
This effectively cuts your cooling capacity to a degree that the engine will overheat on the freeway and at idle speeds.
I always put steel screens (eg. flyscreen) over the radiator hose ends when I first run motors that I rebuild.I just drop the hoses each day and clean all the scale out of the screens. It can take up to a week of driving before all the scale finally stops appearing in the screens.
A lot cheaper than having to have a new radiator pulled apart and have the cores rodded out!
.........good luck
