I always use the core plug heaters simply because it's easier.
On those engines, if you're going to use that heater... I'd probably pull the block drain out below the core plugs on the right side and use that for an inlet. The return I'd probably fit somewhere near the front of the head at the intake manifold. The cab heater return should be there.
I've still got my doubts if that would be as effective as you might like tho.