I think you can come pretty close by measuring the distance the original stud sets in from the back side "outside, trans mounting surface" to the tip of the old ball on the old bellhousing and matching that measurement. If you have a small square that measurement would be easy to make.
If you set both on a flat surface side by side they should measure the same from surface to trans mounting surface.
You might have to compensate for the difference in thickness of the stock and Lakewood plate that goes on the back of the engine if there is a difference.
The concept would be to have the new ball sitting in the same spot in relation to the clutch that the original ball did.
The instructions should be here
http://www.lakewood-industries.com/InstructionSheetsList.aspx?BrandID=5just click on your housing model number.
That isn't going to be much help but they have a tech support link.