I boxed mine from the transition point of the Camaro clip to the rear bumper points with 3/16ths plate. Grind a bevel on the edge of the frame and also on your boxing plate to make a channel for the weld bead. This will ensure good penetration and also make dressing down the welds easier. Definately space the welds as everyone has mentioned, and come back and slowly fill the open spaces until it's fully welded. Use a grinder to knock down the heavy spots, and come back with a sander and put a nice radius on your welded edge. You'll be amazed at how much it strengthens the frame and helps ride quality and handling. Plus, it looks professional.
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Ls650