that's an important point: the thermostat opening is quite small indeed and IS the restrictive point.
one wonders why they attach such a fat rubber pipe behind, going to the radiator, if the thermostat does not allow to flow more
removing the thermostat is one of the things i would try to help with a permanent overheating issue. if your engine reaches working temperature anyway, i can't see any reason, not to try this. this increases flow dramatically
it will give you a hint, if radiator size or coolant flowing speed is your problem.
and, btw, when flushing the system it helps a lot to remove the thermostat at least temporarily.
btw II: it seems that earlier water pump models like the one in my 65 gto had the divider plate, while later ones had it not. this plate divides the pump area into 2 vertical slices, where the inner one feeds the right bank incl. heater and the outer one feeds the left.