With the Hard Top on the Tub, drill a pilot hole for the Hold down Bolt.  Enlarge the hole to 5/16." Insert a 5/16" helicoil into the hole.       Tap the newly drilled hole to 3/8." Screw the helicoil down BELOW the tub lip. I usually brush fiberglass resin into the newly tapped hole before I install the helicoil.  this helps to lock in the helicoil.