Initial Timing Set Up Advise - Inline 6

['68OrangeSunshine]

1 - 5 - 3 - 6 - 2 -4, is GM's Firing Order for all L6 engines.
Tattoo it to the back of your hand or scratch it on the Radiator bulkhead.
As advised by our panel of Experts, as long as your Distributor's Rotor makes contact in that sequence, it's relative position is not relevent. However, by pointing the Rotor in Cylinder #1's direction, you have a bearing. [in a Navigation sense.]

To find, absolute TDC, I use a normal yellow Pencil, and mark lines at 1'' intervals starting with the Eraser. The rubber part hits your Piston top so you don't do any damage. As you turn the Crank with a 5/8 socket on a breaker bar, you are looking for the shortest reading from the pencil.
[Keep that Pencil -- now it's a Special Tool.]

Be advised, Harmonic Balancers have elastomeric rubber to minimize vibration in-between steel parts. Over time, the rubber can deteriorate, and the Timing Marks notched into the HB's circumferance can ''Walk.'' I.E. move off the right mark.
If that's happened with an Over-50 rubber part, Replace the Balancer soonest.