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Brian I'll bet you can find the PRV setup at some of the engine recycler guys up in Michigan or other parts of the country.
I would look for someone parting out a 427, and there are always some of those on ebay it seems, and this is a part they would LOVE to sell someone becuase there is next to NO market for them. If you had a couple of those, it would be easy to bring the system up to original spec. You could put a T in the water pump outlet and be done.