I'm in the middle of my FiTech EFI install and need to find a spot for the O2 sensor.
340 with the stock HP manifolds, TTI 2.5" system with H-pipe

How close or far from the stock collector?
Driver or passenger side, does it matter?

Thx

Well, the stock O2 sensor location on my 87 Diplomat was the in the exhaust manifold between the center and outer ports. Guess they really wanted a fast warm up time.

What does the FiTech installations say? cause that's what I'd go by. Here it is:

Oxygen Sensor Installation
The supplied O2 Sensor can be installed in either exhaust
bank. The Sensor cable connects to one of the cables
coming out of the ECU on the throttle body.
A. The ideal location for the Sensor is 2-4 inches after the
exhaust collector. It must always be at least 18-inches
from the exhaust tip. Where short or open headers are
utilized, install the sensor in the primary tube of the rear
cylinder. Must be at least 8-inches from the exhaust port.
It will not work on "zoomie" style headers.
B.The sensor should be at least 10° above horizontal (see
figure #3) to allow condensation to run off. If this is not
adhered to, the sensor is susceptible to water damage.
C. Never position the sensor on the outside of a bend in
the tubing.

Saw that, but this statement seems geared towards headers and contradictory?

The ideal location for the Sensor is 2-4 inches after the exhaust collector.
It must always be at least 18-inches from the exhaust tip.

2-4" after stock collector?
18" from exhaust tip? Is this at rear of car?

I have an o2 in the passenger head pipe pointing towards the center of the car right behind the 727 bell housing area. Angled so water will drain out of the sensor back into the exhaust. I have TTI 2.5 with stock 340 manifolds. I use it for a/f ratio. Works there for me. Car is 68 barracuda

I think they want it at least 18" in from the end of the tailpipe so atmosphere wont skew the reading. I'd think the 2-4" past the collector would give an homegenized combined AF ratio of all the cyls in that bank. this might be referring to a system with a short pipe extension on the header collector reducer.

The further from the engine the less responsive the adjustments will be.

You want it after all the exhausts tubes merge, be it a header or a manifold.

Collector or thereabouts is best. Mine is a single exhaust on the Dakota. Sensor is at the Y where the collector/cross over pipe merge so it gets an average reading from all 8. Anywhere in the trans tunnel area will work. Stay away from the ends of the pipes where atmospheric conditions could effect it. Also, be sure you have no leaks. That can skew the readings too