JB Weld is PLASTIC and totally unsuitable for exhaust repair.
A proper repair will be done with a MIG (multiple inert gas)or TIG (tungsten inert gas) welder as a catalytic convertor will be stainless steel.
But, catalysts make very poor skid plates for lowered cars, I would get your installation corrected as I doubt there is any room to spare.
Bosch Master, ASE Master, L1
33 years MB technician