I suggest turning the engine in its normal direction of rotation through just under two complete revolutions. Then SLOWLY approach your timing marks, stopping where you want the static timing to be. Turning the engine through its normal direction of rotation without backing up will take up all the slack and backlash in the system.
While I used the Crane/Alison ignition on our GT6 years ago, it was eventually replaced by a Pertronix unit so I don't remember the nuances of how you set the sensor relative to the interrupter that goes on the dizzy shaft. Others will have to offer advice there.