318 2bbl intake manifold?

The exhaust cross-over passage often gets carboned-up and the most practical way to clean it is to hot tank it. There is no way for oil under the intake manifold to get into the cross-over passage. The plate just keeps the oil in the lifter valley from splashing up onto the hot cross-over passage. If it's damaged, I would replace it.

I would also clean out the cross-over passage as it helps driveability for street-driven cars. If you block the cross-over passage to get a cold intake manifold, you will end up with an off-idle bog and will have to increase the accelerator pump shot to compensate. Leaving it functional improves driveability, engine life, and fuel economy.

If you have a problem with vapor-lock, the problem is that the fuel in carburetor is boiling in the carburetor. You can add more insulation (like another gasket) between the carb and the intake which will help. You can also reroute the fuel line away from the front of the engine to more directly over the valve cover. See SlantSix.Org Fuel Line Mod for more information.

Intake Manifold Heat