When you did your Mustang I remember you saying you MIG'ed it all "as it would have taken forever to TIG".
Are you now a TIG convert for sheet metal ?
I've been converting to TIG too, when I can get the gaps tight. The more "visible" it is the more I gravitate towards it, assuming I can get a hammer and dolly on it afterwards. But I'm still quick to grab the MIG when I need something simple or "non-visible". Usually what happens is my panel gap varies a bit here and there, so I TIG everything that's tight, and MIG the wider gaps. My favorite thing about the TIG is the piece doesn't move around so much when you are tacking it like the MIG.
I've been widening my Mustang 4" so I'm deep into this phase (again) myself.
The design is looking good-for an AMX
