you really need to fix the exhaust leak. your wideband is getting very inaccurate readings.
also i took this from my mtx-l instructions.
SpoilerYour MTX-L gauge kit includes a 1” bung. This bung is supplied to best prolong the life of your sensor since it will allow the sensor to sit flush with the exhaust pipe. Using a bung is the preferred method for mounting the O2 sensor for both catalytic and non-catalytic cars.
On CATALYTIC CONVERTER equipped vehicles: Install the oxygen sensor’s bung upstream from the catalytic converter (a bung is included in the MTX-L kit). The bung must be installed in the exhaust pipe at the side
or on top, NOT on the bottom of the exhaust pipe . Any decent muffler or exhaust shop can do this for you. The wide-band oxygen sensor is then installed into the bung to take a reading. The ideal position is between 10:00 and 2:00 position.
On NON-CATALYTIC converter vehicles: You have the option with non-catalytic cars to also use an exhaust clamp as described below. Use of a bung is the preferred method for mounting the 02 sensor for both catalytic and non-catalytic cars.
On TURBO CHARGED vehicles: Install the bung downstream from the turbo but before the catalytic converter. The high exhaust pressure before the turbo interferes with the lambda measurement and the high exhaust temperatures encountered there can damage the sensor.