Best place to install O2 sensor for WB A/F gauge

[Redog]

I just ordered it today. I was on Summit's waiting list for over a year for an Ultra Lite II ANALOG WB A/F gauge, but they called me and said that Autometer isn't going to be making that gauge now, so I bought an UltraLite WB A/F and it's being shipped on April 8th from Cali. Probably will have it next week and I'll have the time to install it on that follow Saturday

Anyway, I was going to remove the downstream O2 and install the O2 there for the gauge, but thinking that was too far down and after the cat, IDK if that's going to be the best spot.

I think I have an extra bunghole (no jokes please) from the Delta I can have welded in the exhaust, but where is the best place to put it?

3400 V6