340 has no balls

Before you check your timing you need to make sure the outer ring on the harmonic damper hasn't slipped.
The only way to do that is pull the plugs (makes it a lot easier to rotate the engine), put your finger over the #1 plug hole and hand crank the engine until you feel compression.
Check your timing mark.
Then, make sure the piston is on TDC. You can use a flashlight to look down the plug hole.
Some people have inserted a small diameter wooden dowel in the plug hole and rotated the engine until the dowel stops moving.
That will get you close to TDC.
Check your timing mark again. If it's within a couple of degrees to Zero on the timing cover, you're OK.
Put the plugs back in and then check the timing with your light.