When I mounted my 120SX i turned up a plug to fit the exhaust and that was glued centrally to a ply disc that fitted the bell mouth. I found it was out so I fitted some 3mm ply as in the pics....

Alan

Also I used a 2.0mm carbon rod for hinge pin (mine was missing in the kit) and when I took out the rod whilst I am re-painting there was no wear at all to hole or rod.