I've use the BMW V12 in my project, but I am using a Porsche gearbox as the Audi will not take the power my twin turbos will kick out. I am told the right Audi is good for 450bhp though.
here is the adapter plate I had made.
http://hometown.aol.co.uk/andycanam/images/cimg1302.jpg
I also know quite a few people who have used the Audi box to the BMW V12 and I think parrell were also doing a kit.
It's not hard to get your own machined up anyhow.