18 to 20 guage is fine.I butt weld my stuff,some guys use a flange tool but but weld is nicer.Mig weld is what i use .Do not use flux-core wire as it reacts to the bodywork.I get my piece as tight as i can,i use 030 wire solid with gas.THE BEST RESULT IS WHEN YOU TAKE YER TIME TO FIT THE PIECE

THE COLD BEER COMES LATER