Nice write up on the Eastwood site about doing metal work without the need for filler at all. All I can say is, :-\
http://www.eastwood.com/blog/eastwood-chatter/how-to-shave-firewall (http://www.eastwood.com/blog/eastwood-chatter/how-to-shave-firewall)
I tend to be more like...angle grinder to cut out metal (introducing heat/warpage), angle grinder to cut out new metal (introducing heat/warpage), weld with my MIG instead of my TIG (introducing more heat/warpage), angle grinder to grind down welds (introducing heat/warpage)...then filler... ::)
It does take some time to blend a panel in like this, but if I can do it......anyone can. The steps they show are exactly right. One comment made was that using the right kind of wire will allow you to use a mig welder to do this - you need a soft wire and don't go bigger than .023.
I also had to learn how to shrink metal when I was doing all the metal work on my old Jag, fortunately I have an oxy/acetylene setup too. Nowadays they have these fancy schmancy shrink discs you put in an angle grinder that heats and shrinks the metal in one step. I did it the hard way with the torch and hammer/dolly.
I learned how to do it by reading books at the library- remember books? ;D