Try it in a daily driver with a stock cam and I bet it will work.

The exhaust system has to be totally sealed up before you can trust anything the O2 sensor tells you. Any tiny air leak and it will read lean. You probably have some header leaks that you don't know about or you have a pan evac system that is sucking air into the exhaust. If you have a big cam then that will usually read lean and if you have a bit of misfire at idle it will read lean.