MIG on low setting, low wire speed. Put a few test panels together to test for depth of weld, etc. Be sure to adher to the warpage rules you use (skipping around to the coolest part of the panel) when you grind as well.

Deve

1950 Chevy 3100 Deluxe Cab
1950 Chevy 3100 Standard Cab
In the Stovebolt Gallery
The Think Tank
More info and tips at Deve's Technet