ive got a heddman header too, and with the 02 sensor location, there is no way the bosch is sticking into the stream at all, i got an ntk/ngk from sparkplugs.com that is identical to the oem one. im getting ~20 mpg highway.

if you want to just work on the bung, i would just cut it down flush with the header so the o2 sensor actually sticks into the stream rather than relocate it.