316 is the most corrosion resistant alloy. Also, heating stainless steel into the sensitized range 800-1400 degrees F or C, can't remember, will cause carbide precipitation which is when carbon combines with chromium to form chromium carbide. Normally a layer of chromium oxide forms on the surface of stainless steel, chromium carbide does nothing to protect from corrosion. The solution is to quench through the sensitized range. Brittleness from quenching isn't a problem with 300 series stainless steels like it is with mild steel because they are in their austenitic state. When you quench mild steel, austenite transforms into cementite and bainite which are both brittle. Slow cooling results in pearlite. Quenching does cause warping and anybody who has welded stainless knows what a pain it is to control warpage. So, instead of quenching, you can buy 316L, a low carbon modification of 316 which you can heat all you want and won't have any serious carbide precipitation problems. When you weld it, use 309L or 310Cb filler. 310Cb has columbium in it which combines more readily with carbon than chromium combines with carbon, so the carbides will be formed with the columbium instead of the chromium and the corrosion resistance of the steel will not be diminished. 310Cb is expensive and hard to find, but 309L is readilly available and works just fine. There are also 347 and 348 which contain titanium instead of columbium, but it serves the same purpose. 347 and 348 are used for nuclear applications.

Even better yet, instead of making an entirely new dash, just buy a rattle can of clear laquer or clear coat and laquer the dash. Do some wet sanding and polishing and it will look great.