How do you adjust timing on a 273?

Before you make an adjustment, put your hand on the valve cover and get a feel for the engine vibration. Now, loosen the distributor hold down JUST enough to allow the distributor to turn. Rotate the distributor counter clockwise until you feel a very noticeable "extra" vibration through the valve cover. It is very pronounced. You will notice it. A blind deaf mute could feel it. Now, back off the distributor slowly clockwise until that extra vibration JUST is gone and lock the distributor down. Drive it. If you don't get any spark knock, leave it there. If you do get spark knock, come back and rotate the distributor clockwise a very small amount at the time until you have no spark knock. Leave the timing light in the toolbox. It just complicates things on a 100% street car.