Hmmm, you need to read my book before you go any further (
http://www.101projects.com).
It seems you are going down the wrong path. First of all, helicoils certainly won't hold up - you'll get pulled studs probably within 100-200 miles of driving the car. It's a waste of time. You need to install case-savers in there.
In addition, the book indicates the proper mounting height for the studs - they *do not* bottom out in the case, but instead are set at a specific height.
Get the book - well worth it when you consider that there are a lot of mistakes to be made...
-Wayne