Remember that the motor mounts are rubber, and the engine will move a little under torque loading. I'd want at least 3/4" of clearance between the fan blade tips and the shroud. Losing a little airflow around the fan is far better than listening to the fan blades creating their own clearance dimension the first time you get on a hard uphill pull.
In my shop, I'd thread the spacer for an Allen head bolt, and finish machine the OD of the bolt head to extend the centering snout, but I've got a lathe handy.
Jerry
