For that crack i'd go with what OOP'S said, take some of the extra from the bob and weld it to cover the bottom of the crack, then you could weld some from the top and clean it up real nice....think that would work well.

I had to weld up a few cracks like that in my cab right under the drivers side seat were it bolts down <img src="/forums/images/graemlins/zombie.gif" alt="" />, the mount for the seat was poking through the floor when i bought it.....fixing that was one of the first mods <img src="/forums/images/graemlins/lol.gif" alt="" />

Good luck with the project <img src="/forums/images/graemlins/kewl.gif" alt="" />, try to post some pics when you're done <img src="/forums/images/graemlins/smile.gif" alt="" />