The block connection is the pressure side of the pump, the pump connection is the suction just like the bottom rad hose. water is pulled in by the pump and pushes through the block. The heater conrtol valve should be on the return to the pump.
That makes more sense as the hottest place would be the water coming out of the block, hence that water would be used in the heater core to heat the car and defrost. The part that seems odd to me (and why I was confused) is why the heater control vacuum valve would be AFTER the water goes into the heater core, but BEFORE it gets back to the pump. Seems like the water flow should be stopped before it ever gets to the heater core instead of the other way around. Guess there must be some really good reason it's that way that I'm not aware of. :-)
The return line is cooler so the control valve should last longer.
Better to control on the return to leave the heater matrix coil pressureized so it doesnt air lock.
Also flow to heater should be at the top connection.