Tim-The 'rivet head bolt' is a threaded fastener. However, it has a flat head that makes them less noticeable, I think.
sstock-Did the rivet-type kit include aluminum rivets? I see that you used a pop-rivet gun for the installation. What was the setting mandrel size? Your grille looks great. Did you have any trouble getting the rivets tight enough?
I was thinking of just using the threaded kit so the grille could be 'easily' disassembled again, if it was necessary.