Roll bar installation - 1967 fastback

I'm trying to install a roll bar into a 67 Mustang fastback, like the ones used on the Shelby's and after reading this forum I have a couple questions. Does the roll bar mount flush to the plates on the floor pan or does it actually go through the plates and extend into the floorpan of the car. There is what appears to be a caged J nut that the roll bar mounts to at the roof line in front of the B pillar. I believe the part number is 381513-s2, does anyone know if that is available anywhere anymore? I checked with AMK and they said they didnt carry it. Thanks