18 gauge or 20 gauge will be easy to bend and will match the thickness of what's already there so you don't burn one faster than the other. Tack the top corners and bend it round then tack the bottom corners and weld all round while knocking the edges down if you need to. I try to get the car as high as I can so I can sit comfortable while I do it.
I wouldn't use 20 gauge for structural repairs, unless the manufacturer used it originally, and will the cars I play with 18 gauge is the norm and 16 gauge on one model (sills I'm referring to, by the way).
Might be a better idea to buy sill sections from someone like hadrian panels.It saves fabricating sections and you can cut them down to what you need.I've bought them for as cheap as £6 each at that price you can't go wrong.If you can't get them for your vehicle sometimes others are close enough to use.
new sills for an integrale are £300+ in italy, and you have to speak italian which i dont, a scuttle panel recently went for £840 on fleabay, I'l be repairing mine then!, the fabricator charged me £10 each for the sills and floor panels, Ant!