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The front will fit fine, but the rears will not be a straight bolt in as the split folding latches are in different places on the Avant
 
I had brackets made up so that I could latch my RS4 rears in my saloon, so I can't see it being too much trouble to do the same for saloon rears into an avant.

You'd be best off getting hold of the latches from a saloon, probably from a breakers yard (or isn't there someone on here about to break one?). Once you've got those, take them to a local engineering firm and they shouldn't have any problems fabricating you some up.

I used the highly technical method of cutting up an old cereal box and marking that to get the required dimensions! I think doing the same for saloon seats into an avant would be even easier because you have better visibility of the back of the seats.

Another point to note is that the bits which go either side of the rear seat (bolsters?) are different for avant and saloons. The saloon ones 'overhang' by about 2" at the back. You should be able to cut them to fit though.