I do like the reverse counter sunk screws....
I mock up the glass,upper frame and posts on the car leaving 1/8" gap under the post base. I then mark the post screw holes against the upper frame, remove the upper frame and run in self tapping screws to make the threads then on final assembly use SS screws. The 1/8" gap under the windshield post is to allow for a drawdown of the frame assembly when you tighten the post bolts under the dash corners BTW I no longer get involved making rubber gaskets from fender welting for under the posts , all I do is made a caulk donut around the post hole in the fiberglass and after tightening down the posts just clean up the caulk that has been squeezed out.
If I have a speedster project with a cobbled frame , I assemble the posts to the frame with SS screws and thin nuts on the inner part of the frame.