The exterior is all different. I'd say the only things that are useable is what's under the hood and the rims.
I've read that the b5.5 passats share the chassis and some other things with the Audi B5 A4s/S4s/RS4s.
If so, does anyone know what we can use on our passats from the audis?
are the seat brackets the same?
Any of the interior pieces the same?
Some suspension from the A4, some from the A6. None of the interior trim, it's closest to the Mk4 Golf/Jetta. Seats are possible, but with work.
My 'A piller back' comment was specific to B5 vs. B5.5, as their body lines are closer, especially in the wagon...
But pretty much anything from an Audi is going to take some or a lot of work to work on a Passat.
How do I make this easy to read......
NOTHING WILL FIT WITHOUT WORK.
NOT A SINGLE PIECE OF BODYWORK
NOT A SINGLE PIECE OF INTERIOR
ONLY, AND A VERY SMALL ONLY, A SMALL PERCENTAGE OF SUSPENSION AND ENGINE.
That being said, with custom brackets, and custom bodywork, sure you can make anything 'fit'.