Most systems that say 'cat back" really mean Axle-back... and they'll fit any ISx50, because it's just the mufflers, tips, and Y-pipe basically... not all though.
Far as I know there's no midpipes that fit AWD.
Mazzuris headers (which haven't been made in quite some time and only a few dozen sets were ever made for the ISx50) do NOT fit AWD.
He custom made one or two pairs for AWD cars, but the gains weren't terrific if I recall. The reason he had to custom make them was the RWD ones don't fit AWD cars. That video is of one of the custom sets he made for AWD.
anything that physically fits a RWD 250 will physically fit a RWD 350 (and vice versa) though in the case of headers you might get poor performance out of it since the pipe sizes won't be meant for that displacement motor.
So to sum up, buy a nice used Tanabe for $400ish and call it a day... or if you want louder get an HKS or something.
It's a 250 AWD, it's gonna be dog slow no matter what you put on it, so buy an axle-back with the sound you prefer if you want to do exhaust mods, you're not getting much beyond sound out of it anyway.
If you're just DYING for a midpipe you might try and check in with the guy who posted this thread though-