If your 'Bird didn't originally have the reinforced rear frame rails w/ the rear sway bar mounts, you'll have to weld a 1/8" plate to the frame to reinforce it where the mount will go. Weld the mount directly to the frame rail w/o doing this and you'll find out the hard way why you need to add the reinforcing plate.
You could also open up the frame rail, install the plate inside the rail, and then seal it up- this would give an OEM appearance but keep the cornering forces from ripping the mount out.
Geno