If you angle your turndown slightly to the side of the cab towards the outside, it should help. Also, where is your turndown located. On my truck I located the mufflers as far back as possible. The mufflers sit just below where the cab ends and the bed starts. The turn downs are located under the bed. When going thru the muffs, I have very little resonance.

Also, were you running cats before and then took them off and the smell started? Perhaps your running rich with the addition of the headers and no cats.

 

That is exactly why I scrapped my true duals dumped at the axle when I put my pacesetters on. I know have a 3" 5x8x18 Magnaflow dumped at the rear corner like you are talking about doing. I like it alot better now. Even witout cats, you can barely hear it crusing at 70. It does resonate a bit under load in the lower rpms though but not nearly what my dumped duals did.
As far as the smell, you may want to look into a tune as mine bearly smells at all even standing close to the tailpipe. Your O2 problems could be causing it too.

 

I have a Nelson tune, and he was supposed to adjust for the headers and such, but i have a feeling thats is why im having problems. Im gonna call him tommorow and have him send me a new one.