1/4" flanges will work.
3/8" flanges are better.
1/2" is Best..
the The Material the flanges are made of is more important.
Mild Steel...ok.
T304....better.
T321....best.
I recommend the TiG process for welding TURBO HEADERS, but you can stick pipe together with a coat hanger and a torch if you want....but I dont recommend it.