I think you should use a Y8 if you can find one, but it's a lot easier to find a Z6. People say the Y8 will give too high compression, but I think you will be all set if you just use the regular pistons. The Y8 is nice because everything pretty much bolts up to the A6 block, and if you stay OBD0 your dizzy will bolt up too. Also, since they came off newer cars, they are generally younger.
I'm running a Y8 head, Dren and Tiersin have Z6s. There are a couple of other people on here waiting to do their swaps too, we have a good support group for when (not if haha!) it doesn't work.