Most high end 6x9” speakers come with spacers allowing a flush mount from underneath or grill hardware for a surface mount installation. As mentioned the package shelf speaker openings are of different sizes, driver side stamped for a 5x7” speaker and mounting screw holes for both 5x7” and 6x9”. One of the mods I did to my car during the restoration was graft in a 6x9” stamping from a donor car passenger side to the driver side of my car with mounting screw/studs tacked in from above allowing easy mounting from underneath.