3003 is the most formable of the alloys mentioned. it's used lots for sheet metal interiors, tanks etc. min bend radii of 1t and (in the UK) cheaper than 5052. 5052 has better mechanical properties but is not quite as formable- more suitable for light duty brackets but can be used just as 3003
normal tempers are 3003-H14 and 5052-H32. if you need to do any complex forming (say, hammerforming compound curves) either can be annealed by coating the area (to be shaped) with soot from an acetylene flame and then heating until the soot burns off. after shaping you'll be back to about half hard (H14) condition.
FWIW 6061 can be bent successfully provided a suitably large radius is used- aim for 3t.
321 is the best stainless alloy to use for exhaust headers, it's less prone to cracking at high temperatures than 304. having said that 304 seems to hold up OK in normally aspirated applications. i've heard some comments that 304 is not up the job (and seen/repaired some cracked 304 headers) but personally i think it's down to poor fabrication techniques
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