I have a 68 Cat that is an X code, 390 2V car that also had AC when it was new. Most of the AC parts were removed before I got the car but it still has the original AC style heater core. The heater hose connections are side by side instead of the more common one above the other routing. My question is which connection is the inlet side? The way the hoses are currently routed, the hose from the intake manifold goes to the outer, passenger side connection and the hose to the water pump comes from the inner, driver side connection. Is that correct? In the WCCC catalog I found a short hose with 90 degree bends on both ends that is used to connect the heater core to the heater control valve. That makes me think the inlet side is on the driver side since that connection is right behind the cylinder head and a 90 degree bend in the hose would make a lot of sense in that tight area and I would expect the control valve to be on the inlet side. The factory shop manual doesn’t help since it does not have an illustration for an AC car. 
After further investigation, I think I found the answer, sort of. I was leafing through a notebook that came with the car and found a copy of an engine compartment illustration for an AC car. It shows the hose from the intake manifold connected to the heater control valve, just as I expected. However, the hose with the 90 degree bends is connected to the other end of the control valve as expected but attaches to the outside, or passenger side, heater core connection. That is not what I expected since that connection is not crowded by the cylinder head and valve cover. The illustration also shows the control valve tight against the firewall which sort of explains why the hose with the bends is between the valve and heater core tube. The other heater hose without bends runs between the inside heater core tube and the water pump. That is tough to do on a big block car due to the tight clearance behind the valve cover. The illustration appears to be of a small block car.
Now for the kicker, does it really matter? From what I can tell without tearing the heater box apart is that the heater core on the AC cars lays flat on its side. In other words, there is no bottom or top. The reason Ford always ran the inlet hose from the intake to the bottom connection on the core on non-AC cars was because heat rises. If the core has no top or bottom it shouldn’t matter which side is connected to the intake or the water pump, right? Seems to me I should be able to reverse the flow which would allow me to use the hose with the bends behind the valve cover and the hose without bends on the heater core connection that is not buried behind the valve cover. Thoughts?
On small and big block 67 68 Cougars with AC the heater valve is located on the firewall just under the cowling “shelf”. The special hose has a 90 degree bend coming out of the outside heater core tube and goes straight up, then it turns and makes another 90 degree bend to connect to the heater valve. The hose coming out of the other end of the heater valve loops under the hood hinge and goes to the intake manifod. The hose from the water pump goes directly to the inside heater core outlet tube. The picture below is a 68 390 with A/C. A little hard to see but the heater valve is in plain sight.
Thanks, John. Your photo and description agree with the illustration.
Do you see any reason why the hose connections at the heater core stubs could not be reversed? In other words, install the pre-formed hose behind the valve cover and connect it to the control valve and the straight hose would then connect to the outside heater core stub and to the water pump. As long as it functions properly that should make for an easier installation. Since the heater core is lying horizontal I don’t see why it makes any difference which side is used as the inlet other than possibly for concours judged cars.
The preformed hose is not long enough to fit from the inside heater core tube to the heater valve unless you relocate the valve. You could go with a 69/70 valve which lays on top of the intake manifod and it’s pre-bent hose does go to the inside heater core tube.