Running lean
First thing I'd do is watch the regular O2 sensors during load for confirmation if you can, normally they'd go into PE mode and peg rich under load, what you are describing they will instead show lean. This will confirm your wideband is reporting/installed correctly and you are indeed running lean. You can use a cheapie ELM327 with Torque for this. The regular O2 test will also show that you're lean on both banks, which could be important.
Fuel filter and/or pump are obvious first things to check if car runs well otherwise. Throw a fuel pressure gauge on it and watch it under load and make sure it's holding pressure, if not find out why. I suspect a clogged fuel filter caused me to harm my original LS1 in my Trans Am years ago, back when I was new and stock and no wideband..






