I believe this is a must do on all BBC after my exhaust valves collided with my pistons on 3 different occasions.
When I switched out my heads from GM's to Merlin's I wiped out valves within 2-3 mins once on plane. (I also eliminated my water pump for a crossover)
I would just dump the water and not run a pressure relief. You want flow for cooling. I have been running this way for 3yrs with no problem.
I also run on Lake MI in 50*F water in the middle of October/November with no issues and can maintain 130*F water temps at idle and 160*F running.
Here is my thread chronicling my ordeal.
Another valve meets a piston - Boatfreaks