You need a water bypass regulator.Available from Greg Shoemaker Marine . Those pumps are capable of putting 100lbs or so of pressure through the engine.Too much for a automobile engine that the cooling system was designed for 15-17 lbs of pressure.Enough to blow head gaskets and intake manifold water jacket gaskets.Especially with a 460 Ford that does not use as many head bolts as a Chevy.
What you are doing is setting the in coming water low enough to let the motor heat up when running but as rpm falls so does the amount of water running through the system.Then it overheats and I got to tell you 220 is too hot!
Buy the regulator,set the water for 180 degrees and it will will dump out the remaining high pressure water at WOT.Must put a resrticter valve in the out bound water to help pressure up the system though.
Does this make sense?