340 Not firing on all cylinders

Wonder if the cam is worn out? Any unusual valve train noises? Oil these days does not support flat tappet cams very well. That is another long story. You do need to purchase a compression gauge. We all need one from time to time. Takes the guess work out of the equasion. Maybe, you can check the reluctor air gap [electronic ignition] maybe the trigger wheel is a bit too far away from the pick up coil. Sometimes by reducing the air gap, you can pick up the cylinders that were missing. .008 I believe is spec for the air gap, and this is checked with a non metalic feeler gauge. as stated earlier, you may be able to "fuzz" the gap some and achieve the necessary signals for all 8 cylinders. just some additional help in different directions which will affect cylinder firing.