Don't have to

Anywhere it will grab, will be fine.
As you are turning the flywheel----all you are doing is checking the runout of the bellhousing.
This measurement will be the same wether or not the indicator is centered

 

Or are you talking about the bell centered to the crank ?
http://www.lakewoodindustries.com/CatalogsDownloads.aspx?brandId=5&ID=102

OH ,and here's the dowel's:
http://www.robbmcperformance.com/products/dowels.html

 

Just stick the magnet anywhere stable that allows you to get the indicator setup right.

I prefer to do it without the disk or pressure plate installed, and stick the indicator base magnet on the flywheel's friction surface.

 

Thanks all! :thumbsup: