Yeah, it's easiest to put it in the existing rear bung...I asked about this a while back and got enough people that said you can just use the existing bung for the wideband.
I wrapped the hardware itself around the subframe connector on the DR side (with extra large zip ties), and routed the wires for the gauge and power up thru the shifter boot.
The SOS gauge pod is the bomb - it's pricey, but it looks the best, IMO. I'm planning for the same when I install my wideband and FP gauge back in my newest Mustang. (Be sure you get it for the 03-04 pillar...the 99-02 was much thinner and wont work in your 04. (New rollover restrictions came about in 2003, hence the beefer pillar))