That's the Tremec 6060, correct? There was some discussion on the Viper forum about putting them into late-model Vipers. In the discussion, they mentioned the difference in the gear ratios of 5th and 6th gears between the Viper 6060, and the Caddy CTS-V 6060. I wonder where the 5th and 6th gears of the Challenger 6060 sit.
That's awesome - I wish you luck with the project. By the way, T-6060's aren't really hard to get - it's not like Chrysler is building them. They're just selling you a part off of Tremec's shelves. If the Challenger and Viper 6060's are the same, then the Caddy's has a shorter 5th and 6th gear for better top-end acceleration (and thus lower fuel mileage at cruising. *shrug*)
Good luck!!
Jeff