The 6013 rod will work, but something tells me you arent too familiar with stick welding? You may want to practice on a few pieces of similar thicknesses. It will certainly work just fine, requires patience, and skill. The 1/4 stiffeners are thicker than the unibody, so you will want to concentrate your "puddle" on the thicker material and work your way over to the thin stuff slowly. 90 amps is a good starting point. Make sure the rods are very dry, and the material is very clean, it will help with rod sticking, and striking an arc.
Overhead is simple enough, just takes practice... and a good thick welding apron/jacket/tough skin...LOL...
Good luck with it.
~James