As I said in an earlier post the moulding has a big sweeping curve and I thought 18 inches from center would yield a far bigger number but it only added about a 1/4". So in the center measuring as in my drawing the C it's 20 1/2 in the very center of the bow vertically side to side on the car (as well as center horizontally) and then add a 1/4" if I moved that out 18" from the center.
This is all with a perfect 17 5/8" upper measurement exactly as the GM manual says.
I like your idea of hanging the top, I was thinking of that myself. I won't be able to until this weekend. But it will get darn hot in that garage tomorrow.