Mounting New water pump on slant 6

That plate does not make a pressure seal, so no gasket needed. Its purpose is to direct the water flow. It is typical for water pump bolt holes to penetrate engine chambers, like crankcase (oil) or coolant (water). Check each one with a wire to see if blind. If not, you must seal the bolt threads. I use "gasket sealant", which is a liquid goo that stays flexible, and cleans up with ethanol. To temporarily hold the metal plate in place, you could use vasoline, a few dabs of RTV, or maybe bearing grease. I would probably use RTV (the day before).