does it matter where your primary o2 sensor is as long as its before the cat? would right where a cat or resinator flares out at the intlet be ok even if its under the car halfway? i didnt wana shove it in my 2.5" piping cause it might block flow a little

 

generally, 10-12 times the diam of the port (or exh runner) = where to put the o2 sensor downstream, lengthwise.

i really dunno about this, people put them everywhere.

dont mount them where water and fuel can lay in them. always point the electrode downwards.

 

right now my primary is under the firewall stock ls exhaust, isnt that too far according to that?

 

Dyno shops put the wide band just inside the tailpipe... Non-optimal, but it works. In other words - it's probably fine in front of the cat somewhere.
EGT sensor.. that's different.