Cut right in the middle of the up stream weld. Remove all material. I think the cat pipes are 2.5 so take a piece of mild stainless. Make sure the opening in the cat shell is big enough for it to slide in. Mark the end to be cut on your piece, and pull it back out. Now measure that distance to the down stream side of the case, and drill four 3/8 size holes, equal distance apart. The holes should be in the weld. With the piece cut flush, stick it into the case, and weld the down stream holes shut with stainless welding wire, and smooth out with angle grinder. This should catch the down stream side of the pipe. Now reinstall both pieces of the cat back on the car. and tack both together in several places. Remove, finish weld with stainless wire, and reinstall for a final time, and check for leaks. Do make sure that what ever material you remove from the cat case to enlarge the hole to fit the pipe is added to the end of the pipe to make up the difference.
2014 Chevrolet Silverado LTZ Z71 5.3 Aluminum V8 (My daily)
2007 Pontiac G6 VL 2.4 I4 (Wife's Daily)