Before you mount the center clamp, be sure that the top bow comes down easily on the header. The studs should fall into the holes in the side brackets. If this doesn't happen, it is likely the top will have to be adjusted using the eccentric bolts at the sides. This is a lengthy process, as any adjustment affects all others. The seal on the bottom of the header should make full contact across the windshield, but not be so thick that excess pressure is required ob the center clamp. The serrations on the bracket and the clamp will allow fine adjustment. I have yet to see a replacement header that doesn't need fitting. If the top is already covered, it prevents that part of the job.