Heated water rises, so that's why you are supposed to install it in such a way that the heated water will rise into the engine, rather than be wasted in the radiator.
It looks like the hose drops down ahead of your intended heater location, to the bottom of the radiator, so I believe installing the heater where you propose would work... the heated water would move to the block rather than the radiator.
That being said, why not use a block plug heater?
I just looked at my Zerostart catalog... their part # is 3100013.
Book says it installs in the right side of the block, with the element pointed down.