The exhaust manifold has 10 layers of wrap and UEGO sensors don't like it too hot, located back there it's in the correct working-temp range.
There's more to be done to the exhaust yet, but that job alone was just one straw short of breaking the camel, but the most important job has been jobbed.
I'm going to reduce the tail-pipe size from 3.5" ID down to 3", fit another EGT sensor to the front of mid-pipe, make a new branch-section incorporating a small-resonator, then re-fit the OE heat-shield and tunnel-brace.
New 3" bore 4" OD stainless-steel tail-pipe
AEM EGT gauge sensor and fitting
Present (and failing) Y branch section
Donor Skyline 3" bore stainless-steel resonator