Just like checking for a bad coil on a multi-coil engine. Swap around the coils and see if you have the same fault. Swapping a rod and piston will give you a clue, if it is a rod or piston fault. Sizing the rods may be in order. Also I would check with a plastic gauge on the bearings for any slop. I know the early 340's are high on the block, so the one that is lower may be a fault in a loose bearing.