When you pressurize the system from the turbo intake, you have to keep the air hooked up because air is constantly leaking out through the engine. You're bound to have at least one cylinder where the cam is on overlap - both valves open at the same time. You lose some air, but your compressor should be able to keep up.

You can do it as you suggest and block both ends, but that does not test the gaskets for the grids and the intake manifold cover.