which way is the water flowing in the inlet manifold ?
into the block or out of it ?

Out

thought so, so if my heater matrix is going to be horizontal, i would be better off with the water going into the bottom hose

I would say yes less chance of air locks

As a rule water in radiators feeds top to bottom. So as it cools it naturally flows down. Applies to both cooling rads' and heaters. The engine is the opposite and flows bottom to top, rising as it heats. Generally heater return to the bottom hose or direct to the pump means your not circulating the water through the main rad before the stat' opens. That way the engine comes up to temp quicker.