My first bracket design was for my son's '74 Dart Sport to add a pair of Volfenhag 3.5" two-way speakers. Those speakers have two features going for them. A built-in raised ring allows the speaker to mount flush to the bracket's bottom surface without making contact with the flexible rubber surround. They also have low-profile center tweeter cones which avoid making contact with the underside of the dash grill. This all makes for a very compact installation. Of course, this was in an A-body dash with plenty of room below.

If clearance is limited in your particular case, top mounting your speakers through the bracket openings would gain you approximately 0.080", the material thickness. Slipping washers or thin spacers between the bracket and speaker mounting flanges could provide a little more lift, but the limiting factor then becomes available clearance above the center cone.

Choosing the right speaker is critical.