all the cats i have seen have a 3-4 inch flange forward of the cat. many end in a flared lip. the header pipe has a customized end as well.

all this fancy metal work on each of the mating pipes make for awkward fitting and sealing.

the dirt simple country boy cheap and durable fix to all this is to reciprocate saw off the fancy ends off each of the pipes leaving a plain vanilla pipe end on each with a gap of an inch or more between.

obtain a filler pipe sleeve which will slide over both ends of the pipes and then exhaust clamp each end fo the sleeve.

job done.... quick fast and clean. any future work simply requires a saw cut through the sleeve and dremel slitting the the remains. you will get a perfect seal easily using this method without the aggro of tweaking yourself for the right donnut or the right header or cat or whatever.