340 Not firing on all cylinders

Use a spare spark plug and place it on the coldest cylinder's wire, then start the engine. If you see it firing, then you can suspect a lack of compression. Place your thumb over the spark plug hole in the head while someone cranks it over (remove the coil wire from the distributor first). See if there is a lot of pressure trying to blow your thumb off of the spark plug hole. If not much, then you may have a hanging valve or broken piston rings.

Good luck!