I just welded a new corner in my cab and a pair of fender patch panels. I agree that the crimping method with its overlap is a good method, but I chose butt joints as they do not have any overlap that can retain moisture. (You might be able to seal this up by also welding the inside.)

My contribution to this thread is to mention the Eastwood (sponsor plug) tool that clamps butt-welded panels together. They come four to a set and hold the metal panels aligned at a nice gap for welding. My only difficulty is that one must space the gap a tiny bit more than the thickness of the tool's blade or it will be difficult to remove without doing minor damage to the tool (ask me how I know).

P.S.: The air blast cooling is a good idea - - I'll use it myself in the future to speed things up. You will not harden the low-carbon steel panels, but do not cool too agressively or you will create more distortion than you want.