Be careful when buying them. Some are absolutely shit and have fallen to pieces, others are a real work of art.
Stan "The Man" Pobjoy uses only the Al case for his "Steroids" motors now, so he would have a supply of them I'm guessing.
A lot of the yanks are still saying that for anything other than a pure performance engine a new AS41 case will work fine. CB Performance sells a "super case" which is an AS41 with some work put into it. Good deal.
http://www.cbperformance.com/catalog.asp?ProductID=185
Give Stan a ring and see what he's got, and what he reckons.
Be warned though, he's a good talker.
PS
Here is a link to a real work of art:
http://www.precisionalloy.com/pages/tf-1.html
[Edited on 11-6-2004 by 56astro]