I butt weld most but bought an inexpensive air flanging tool. Works very well on most sheetmetal & makes those welds in hard to see places go so much easier. I made sure to seam seal after I was done because you'll be leaving a real nice area for rust to start if you don't.
Most sheet metal I worked with is probably 18 guage or thinner - not sure where you'll find 16 guage on most trucks.
