Where to mount wideband O2 sensor.

You want the O2 sensor in a spot in the exhaust where it won't get too hot (less than 900*F I think) but also where the exhaust isn't too cold. The O2 sensors are heated, but the closer they are to the correct temperature the more accurate they'll be and the less the sensor heater will have to work (don't run them unpowered in the exhaust at any time!). Further back in the pipe condensation can be more of an issue too, you don't want the sensor loading up. You're also supposed to mount them at an angle, again, so nothing liquid builds up on the sensor.

The other issue is the header flange. Ideally, you don't want your o2 sensor downstream from a potential leak that could throw off the readings. So, in the header collector a few inches ahead of the flange is probably ideal. But most folks don't want to weld a bung into their brand new coated header. I know I didn't. Mine ended up about 8" or so behind the header flange. You also have to contend with the length of the sensor harness, the angle on the sensor for clearance, and in my case, 4 speed shift linkage rods, big torsion bars, etc. So mine ended up just behind the shift rods, ahead of the crossmember, with the sensor harness going up the inside of the 4 speed hump and through with the LC1 under the driver's seat.

Seems to work well, I run the same innovate LC1 and Bosch O2 sensor. But mine is just for tuning and diagnostics, I run a carb so the sensor doesn't actually control anything.