I know that DC Supercars have also fitted BMW V12's in their Diablos/Countach's and i'm sure they used Audi boxes?
I saw a BMW V12 in a red Countach replica when i went to visit them a few years ago and it looks awesome! Now there's a word i haven't used since i was a teen, but is appropriate here!
I'm sure there is some info about this over on the Lambo forums or someone over there will know?
http://www.lamboreplica.co.uk/
Good luck!
Dave