My headers are VERY close to my steering shaft. I had to clearance the primary with a hammer. They are stainless, so I also DEI heat wrapped them (for many reasons) which provides a little cushion if they happen to touch during hard cornering under acceleration. I wouldn't have done that with mild steel. I would keep cats
at least an inch away from the trans case, but 2"+ is ideal. Chassis/frame rail I wouldn't be too concerned with as long as they wouldn't touch under any circumstance (and you didn't have some pretty finish on it that heat could melt or tarnish). When in doubt, heat shield it. I like the DEI heat shielding products. Titanium header wrap, trans tunnel, etc
http://www.designengineering.com/cat...-sound-barrier