There are many here smarter than I am but my research found that the best placement is as close to the cylinders as possible. In fact, newer hi-performance vehicles put them right on the exhaust manifold one-per-cylinder.

As for directional placement, I found some research that suggests you should avoid up and down configurations and placement at 11 O'clock or 1 O'clock is better.

On my Ebody we "hid" them down near the transmission. You can't see them unless the car is lifted. As much as I hated defacing my Accurate stainless exhaust, I hated even more the idea of tearing apart the exhaust to do this work later.

I didn't place them as close to the exhaust manifolds as I would have liked but I believe I got accurate readings here. We put on on each side.