Hi Frank,
A few things come to mind that I've not seen mentioned above, so here goes my addition to the composite wit:
1) is the water pump turning fast enough? It is good to have the water pump pulley diameter/crank pulley diameter ratio up to snuff. It is somewhere around 1:1 although the pump pulley can turn a bit faster or slower than 1:1. As examples you might want to look at the March web site where 460 crank pulley / water pump pulley size pairs are given, such as (crank/pump) 5.75"/6", 5.5"/6.25", 5.75"/5.25" and 5.75"/5.5".
2) is the water pump turning the correct direction? Some are made to run opposite due to being designed to use serpentine belts. Check part numbers with Edelbrock. If it is turning the opposite way from what it is supposed to turn, its output will decrease a LOT!
3) Ford has introduced a new water pump with extra capacity ("MAXIMUM FLOW") for the 460 - part number M-8501-C460. It is now standard on the 460 crate motors. You don't suppose there was an overheating problem that caused them to make this change, do you?
My 521 does fine with the above pump, unless there is a big air bubble trapped in the system
Let us know what you find out!
Regards,
Tom