1968 340 timing

^^X2^^

I'm not familiar with those marks someone added them? OEM timing mark should be one big deep slot clear across the balancer

Timing light should be on front driver side

Timing marks/ balancer/ tab is different from earlier (pass) to later (driver) side timing tabs. you cannot mix those balancers

Lacking anything else, take a vacuum gauge on manifold vacuum and time it for highest idle speed, then hook up light and "work backwards"