[Formulajones]

Usually the car dictates it to some degree. Easy access for future maintenance is one thing to consider.

You can put it as close to the flange of an exhaust manifold as you want so long as it's easily removed/installed. Down the pipe several inches doesn't hurt anything either.

On header applications I put them in the exhaust pipe after the header collector flange.

In the Holley software there is an adjustment for the 02 if you end up down stream a ways, that affects how fast the 02 reacts, so you don't get an AFR reading that is jumping all over the place.