I think they want you to run hood pins. If the Mustang your putting it on is the one in the pic, I would get the shelby valance that allows you to keep the bumper.
I cut off the bottom section. I had fabricated a tie point with some flat stock down to the front cross frame, but the A/C condensor got in the way of that so out it came. Not what I would perfer, but there are not many options other than hood pins. I put the patch on this weekend. It seems to latch ok, solid enough. I adjusted the strike and the latch to be as smooth as possible. It clickes right in and does not seem to move the grill frame at all.