Hello monsoon12, using a back-step method will definitely be better than the first method that you mentioned. Also tack-welding at every start/stop point that you plan use to accomplish the back-step method is a fairly common way used by many fabricators to gauge the length of these welds. If you want to further help with keeping the SS plate from ending up bending and warping you should use strong-backs that run across the SS plate 90 degrees to the weld line and spaced apart from one another on 6" centers or so, the more you use the straighter the piece will end up being. If it is important for the beam to stay straight also, you might incorporate another strong-back along the web-line of the lower flange of the beam or plan on doing a little bit of heat shrinking to straighten it out after welding. Almost forgot, try to use a minimum size fillet weld to further limit the amount of warpage. Hope some of this made sense and good luck on your project. Regards, aevald