The 425hp engine is a 6.1L. The 6.6L is a rumor, but I will try to find the page. I would say it is more likley that a 6.4 or 6.6L engine is placed in it, because 425 hp for 40K+ is not going to do the job verses the the near price, 500hp camaro and GT500. The camaro is a rumor also, but i'm sure GM wont let the GT500 steal the light.
Edit
Here is the link
http://www.leftlanenews.com/2006/07/...008-challenger