I've spoken with several people about a mock-up case. Tremec charges some ridiculous amount for a case. The other option is to pick up any T-56 case and bolt a Magnum tail-housing to it, but they are also stupid expensive from Tremec. I'm going to just go ahead and buy my transmission when the time comes (I guess).
The only other thing that comes to mind is this item from Speed Tech.
http://www.speedtechperformance.com/...rod/prd264.htm
Aside from that, I'm not sure if anyone else makes a "pre-fabbed" tunnel modification solution.