With the injection wiring squared away, I drilled a 7/8-inch hole for the oxygen sensor downstream where the two cylinder banks meet. You don't want to install the sensor too far downstream or there won't be enough heat to get a proper reading. Generally, as long as you're forward of the bellhousing, you're okay. Although you can weld in the supplied O2 bung, Professional Products designed this system to be installed by most home hobbyists, so I went the low-tech way and used the gasket and hose clamps as shown. The bung is designed for smaller primary-size tubing and my 3-inch-diameter pipe is a bit too big to fully seal. I'll be welding the bung on in the future to eradicate the very slight puff of exhaust that escapes past the gasket.