Try to get the O2 set about 3" behind the collector
end(where the primary tubes ends).. you want the
air thats coming out of the primaries.. put them
in ahead of the breathers so you dont get a bogus
reading... also make sure they are angled downward
so they can drain any moisture.. you want them
below 9 clock and 3 to drain
EDIT
there isnt a true AFR reading.. for cruise you
would like in the lower 15 range..for WOT it
somewhere in the upper 13s.. I dont use them to
get a idle reading(I want the best idle I can
get thats clean)your car may want numbers a
little different