[Kevin in NJ]

I have learned TIG welding on my own. I have a Miller 330 A/BP that can use more current then my main box can supply.

I am NOT anywhere near an expert on TIG welding. I have fumbled through it all and now do some really nice welds, most of the time. It did not help to have a 1962 vintage welder with the start run relay wired backwards making the foot pedal not work right at first.

For welding steel you need Thoriated Tungsten, the red ends if I remember correctly. You will want the .040 (smallest diameter? I am doing this from memory to late to go to the garage) for sheet metal, the next 2 sizes up depending on the current the unit can supply. The diameter of the Tungsten will limit the current that will flow.

You do not have AC, but if you did and wanted weld Al then you would need plain Tungsten.

Use the .040 for the sheet metal. You need to have clean metal. TIG is not very happy with rusty metal. Get the ER70s-6 rods in various sizes (I forget the size I use) and MIG welding wire works too. I have used some .024 mig wire for some welds. Practice some. When welding you will try to minimize the amount of filler you use. When you make your patch try to get a real tight fit. The Silicon and other trace elements in the rod help to move the contaminates out of the metal. You will find a very small amount filler rod will act like when you add flux to solder, the steel with start to flow better.
Hopefully you have a HF start and a foot pedal. You use the foot pedal to get the melt started and then back off a lot. You move the foot to control the heat. Push down to gain some heat and then back off to move the puddle or so. It works real nice. Take it easy as you work up to the edge and just let the puddle flow out to the edge as take the heat off.

I suggest that you get some metal working videos. There is one on making a fender and another called Shrinking Magic. They show the process of doing a few welds and then doing the hammer on dolly hits to correct the shrinkage.

For the shells, it is THIN metal. You need the 318 or 316 filler rod, the real thin stuff. They make it like .020 or something. I have a bunch of 1/16" 318 stainless and that is too thick. It takes too much heat to melt the rod and you just blow holes in the shell. I even tried to hammer it thinner. My local welding shop did not have any thin stainless rod in stock so I went to a local welder and he gave me a bunch of the real thin rod. That made a huge difference, but I have not had the time to improve my techniques. Keep in mind that every weld is a shrink and you much make corrections on the thin stainless.
Welding the stainless is only the start. Yo also have to level and remove all the scratches you have put into the shell. You will have a lot of time invested in doing it right. Not to mention you will find the expanding rubber wheel with trizact bands are a big time saver (see Eastwood).

Anyway, it is not rocket science. You just need to poke around the net some for the technical details. Once you settle in on some practice welds you will get comfy real quick and will be ready to conquer some major projects.