Smoothy running boards

I was just wondering if any of you guys have made smooth running boards out of a stock set, the ribbed tops of mine are rusted through, and i am going to weld in a new piece of metal, i was thinking about cutting out the center and welding in a new piece, or fitting a new piece over the old top and welding around then grinding the welds smooth making a slightly thicker running board. Let me know what you guys have done and any tips you have for this.