Gee the time spent looking at this could have paid for an O2 sensor. I realize the heated ones are a bit more than the $19.95 Bosch sensor for my 87 Buick LeSofa(er I mean LeSabre) but can't be too much more.
One other, not so smart @$$ comment. The O2 sensor after the cat IS tied to the upstream sensor, IIRC. I think the folks who make the MIL eliminators for those who've done the "Mystery Mod" use this information to craft the circuitry for that item.
Check out this page on how to build your own MIL eliminators.
http://www.eaton-design.com/mustang/mil/ Now I've spent enough time here as well.
OBTW, another reason for code P0171 is a dirty MAF, I think Lee Cox was having problems with that. Some brake parts cleaner on the body and some electrical contact cleaner on the wire should clean that right up, FWIW.
TB
