Having, or having had, Caterpillar 416, 416C, 426C, and 420E backhoes, I dont ever remember having a coolant leak anywhere. I do remember the space between the radiator and oil cooler being plugged with straw and debris enough to cause overheat and boil over.
Also, there is no coolant recovery bottle on there, so, the radiator is not to be filled to the top. If filled to top, coolant will come out the overflow hose until the level is back down to normal.
It has been a while, but, I'm pretty sure that when looking in the radiator fill hole, there will be a raised block in there. Top of that block is the cold fill level.