rollingon15s
I would go with Bonnevillet140v's advice and pick up the full kit. There's a lot more steel goes into the back of the ST to carry the pannier rails (which are plastic).
Here's a link to a page that has the fitting instructions, and the diagrams showing all the parts used...
http://www.triumphnet.com/
Use the links menu on the left to select the following options and you will find the instructions
99-04 ST > FAQ
at the bottom of the page you should find "Accessory install tips" and then on the next page "Triumph Top Rack /2 Box Pannier Set"
these should get you there!
Rexx