340 Not firing on all cylinders

You should GROUND spark voltage if you pull a wire out of the distributor. This is not about getting yourself zapped, it's about not crossfiring open circuit spark voltage, or causing damage, here's why

If you are "testing" for cylinders firing, and pull a wire off either at a plug or the distributor, the open "spot" in the distributor goes to VERY high voltage when the rotor comes to that disconnected wire and fires.

This can kill an ignition module and there is caution about this in modern shop manuals

This can cause crossfiring to other cylinders which will make your test results improper. In other words you can cause a miss in ANOTHER cylinder while you are attempting to evaluate the disconnected cylinder

Especially with modern high energy systems, this can drill holes right through caps and rotors, or carbon track them, and otherwise damage secondary components

So when testing "cylinder balance" by disableing cylinders, do this:

Either stick small thin brads down beside the wires in the cap, so you can just ground them with a probe, or.........

Use insulated fuse pullers in one hand and a grounded probe in the other. As you lift each wire out with the fuse pullers, slide your grounded probe in under the wire so the spark "has a place to go."