Xack,
Not sure I understand the problem
I have a 521 in my Cobra and a 557 in a Zephyr wagon. Both have stock Edelbrock water pumps (sorry no photos) which seem to move enough coolant to prevent major problems.
Both are extremely sensitive to trapped air in the system. The Zephyr has a smaller radiator and a very restrictive engine compartment (read: overstuffed) so it has a bit more load than the Cobra.
Each has a thermostat, either 170 or 180.
The Cobra will idle literally for an hour on a 98 degree day with NO problems. The Zephyr will too as long as the fan runs on high speed.
Both engines run 170-180 on the highway and around 200 in traffic.
If yours runs 200 or less, maybe there isn't a problem?
Also, I believe a thermostat is a necessary flow restriction. If there's no thermostat, so the conventional wisdom goes, the coolant passes through the radiator too quickly to exchange its heat.
460s are particularly sensitive to the block plate that fits between the pump and the timing cover. Since your photo shows the plate I think that's OK!
Hope any of this helps,
Tom