340 Not firing on all cylinders

are the wires to the ignition module properly connected to the distributor? Sounds like it could be something 68gtxman mentioned, maybe the reluctor gap is too large or the contact points in the cap might have broken off, unless you forgot to put the rotor back in LOL

Do you have a timing light? If so, you can hook it up to any plug wire and see if it flashes when you try to start it, easier then grounding out a spark plug :)