If the leak comes from the area between the catalytic converter and manifold, most likely it is due to a bad exhaust system alignment. Below is the correct procedure to ensure no leaks:
Beginning at the front of the vehicle, align the exhaust system in the following sequence.
Evenly tighten all 4 of the catalytic converter-to-exhaust manifold nuts (starting with the right lower, then the left lower, then the right upper, then the left upper) in 10 Nm (89 lb-in) increments until reaching the final torque of 40 Nm (30 lb-ft).
2014 Mustang 3.7L cyclone engine with 6 spd transmission. Mods: 93 MPT Race Tune, 3.31 Gears, Airaid CAI, BBK Shorty Ceramic Headers, Corsa Sport axle backs, Strut Tower Brace, Steeda rear seat X-brace, BMR Rear LCAs, GT wheels, MGW race spec shifter, Passenger seat delete, spare tire delete, Moroso battery relocation to the trunk.