Heater hose routing? It doesnt matter.
The hottest water IS exiting the intake manifold on its way to the top of the radiator then the water flows through the core just like a small radiator to heat the air provided by the fan. It doesnt make any difference to the core which direction the water flows.
Water flows ONLY when the valve is opened. (temp lever slid to high)
Now, does direction of flow matter to the VALVE???
Not on our trucks. Take your valve out a look at it. Whether metal or plastic, the valve opens & closes a "gate" that is not affected by the direction of flow.
SOME vehicles have a heater valve (commonly vacuum operated) that forces a 'washer' against a seat much like a [censored] faucet. These valves are held closed by a (weak) spring and opened by a small diaphram.
IF the flow is 'backwards' on these valves THEN the seal can be forced off its seat and hot water can flow thru the core even when it is turned off.
Again, this situation is not an issue for our trucks
The 'water cooled' carb spacer Fordman was discussing is supposed to be connected to the HOTTEST water. (from intake manifold nipple then to heater). It was included by FoMoCo to 'prevent carburator icing'.
Expanding air (lowering air pressure) as in comming out of the venturies is absorbing heat and could form ice on the throtte plates resulitng is poor cold (freezing) weather operation. The carb spacer was intended to HEAT UP the carb!
Out west it is common to re-route the hoses to NOT run water through this plate to prevent vapor lock.