blocking manifold exhaust crossover?

Your intake gasket set should come with plugs to block the crossover.

The crossover is supposed to help warm the intake charge faster on cold mornings in colder regions of the continent for better combustion, lower emmisions, smoother idling, etc. Of course a warmed-up motor requires a colder charge for better performance. They made a calculated compromise between performance and user-friendliness by putting in the crossover.

No, you don't need it. No you won't hurt anything by blocking it. Yes your car will run better with the intake you're using.