on my application i used 3/4 from the water pump heater core line and made new line up to the heater core then i T'd off with 3/4 down and back to the head. I used to good blue silicone hose that was able to stretch to the 7/8 to reach over the output of the water pump and suck down to the coolant heater. The Espar is a goody metric hose size that 5/8 standard stuff won't go on, and 3/4 standard hose can't shrink enough. so the good blue is a must.
either way you run it, you need to map out the natural coolant flow path from pump to head/block. Its important so that you can have the engine running and the espar running together and not trying to reverse flow the espar and have it kick out. Or have the flow path from the water pump starve off the espar and throw and overheat code.
other ways i tried were going was nearer the middle of the head on return path, and i pulled from the outbound line of the cab heaters (inbound line now). that didn't give a consistent heat and made it hard to keep truck warm while driving around down an idling at -40 up here. my new path allows me to keep about 80c in town if its that cold.