Forgive my noviceness. When using the light gun on the mustang 289 is the mark I am shooting for the 6 left or right of tdc

 

Assuming a normal motor, and not a marine one that runs backwards, as you face the engine from the front of the car, the engine turns clockwise.

So, if TDC were to be directly on the top of the crank pulley, 6 degrees BTDC is on the right of that mark, in the direction of the crank turning.

I'll say "right," but, it's really a perspective thing. It's more "in the direction of the crank's turning., which could be down, or up, or left, even, depending on how you're looking at it.

/Not sure I just confused you... I'm so sorry. Go with 'in the direction of the crank turning.' and that will always be correct, on any motor.

 

One side of the harmonic damper is marked clear up to 30 degrees before top dead center. The other side of "0" is after top dead center and usually only goes to 10 degrees after top dead center. Set your timing to 10 degrees BTDC even if it calls for 6 degrees BTDC. It will run fine there.