Just a note: If the EGR valve is stuck open, disconnecting the vacuum hose and plugging it won't make any difference.

You would need to remove the EGR from the manifold, and put a plate over the two holes on the intake to close them off.

Replace the plug wires, you have no idea of their history. They can develop breaks anyplace along their length and short out the spark to a different wire or to a ground source. I've even seen them short out inside the plug boots!

Just for the sake of weird, I found an old big block that had a inconsistent miss, the plugs, car rotor and wires had been changed several times chasing the miss. What I discovered was there was too much antiseeze on the plug threads for them to be a good ground for the spark. I cleaned the threads in the heads and the threads on the plugs with carb cleaner, dried everything off, added just a touch of antiseeze and reinstalled the plugs and the miss was gone.

If your 318 has evidence of oil surrounding around one or more plugs, it might be worth cleaning and drying the plugs and the heads around the plugs, add just a touch of antiseeze, reinstall the plugs and see if the miss goes away (after you replace the plug wires). Gene