Hi all
Just a question as a fellow Datsun friend and I are disagreeing on the matter although I think he has a point.
My cabin heater core tap got stuck due to rust and age in the closed position. I then decided many years ago to bypass it in the engine bay and neatly put plugs in the firewall where the pipes would have entered.
It is currently connected as follows: pipe from the BOTTOM radiator hose is joint to a T and then enters the cylinder head at the back. So radiator water permanently flows into the back of the cylinder head. As you know, when the cabin heater is activated, radiator water will anyway flow this way. My friend say I must rather block the cylinder head at the back and also the radiator T piece as it is cold water entering the cylinder head while hot water from the engine block enters the cylinder head from below. This might cause a cold spot at the back of the cylinder head. I believe he has a valid point but can it cause harm? Must I rather block so that water only flow from engine block to cylinder head?

You have it backwards the lower radiator hose is the suction side, remember heat rises so the discharge will be at the upper hose, that's why the thermostat is in the cylinder head. They didn't change to reverse flow until the 90's.
Leave it the way it is it won't do any damage.

You can do it either way

* Route heater tap on cylinder head back to heater pipe (just without heater). As you note, this is how it works from the factory when the heater is open (lever fully to HOT), back of cylinder head is connected to radiator pipe

or

* Block the tap on back of head. This is what Datsun did for STD 1200s without the optional heater

Nissan part number 00931-30610 PLUG-HEATER OUTLET HOLE


Heater hose plug for cylinder head (as fitted to no-heater models)

Plug thread is 3/8" BSPT

Don't plug it. Route the rear of the head back to the front with a short piece of heater hose. If you don't, youll create an air pocket in the rear and the engine will overheat. Keep the fluid flowing. 👍

Currently the water is flowing with a heater pipe connected to the bottom radiator pipe with a T piece. Same as if the heater would flow in winter but with no heater core.

That will work no problem. Any air in the system will bleed out the top. When refilling coolant just be sure to bleed the system as with a normal heater installation

What you don't want is to block the hose, which can create an air pocket. On the other hand, what Nissan did was block the back of the cylinder head which is OK if there are no heater hoses (as in a STD 1200 with no factory heater)