That looks like a carburetor spacer. It is designed to give you more horsepower through higher velocity air flow. This would not cause a miss even if it was leaking. You seal this spacer with gaskets, top and bottom. The miss is caused by your spark plugs being corroded. I say this, because you said you checked all of your wires and did what "dancytron" said. (The easiest way is to let the motor get up to temperature... meaning, hot water is felt through your upper radiator line because the thermostat just opened up. Then turn the engine off and pull one plug wire. Now start up the engine and see. Let it run for a little while. If it is the cylinder causing the miss, one of these "unplug wire tests" should make a major difference, unless you have drivetrain issues in multiple cylinders. In that case, a good vacuum test and a compression test should show it. If it doesn't, I would check electrical again.) Remember, the miss should be easier to detect at idle... the engine is making less noise, so you can hear it better. So after removing a wire and then starting the engine, let it run for a little while. Mainly to let the idle come back down.