There's one and only one cleaner that has been shown effective and safe for doing this task, and that's Auto-RX.
Other cleaners tend to break off large chunks of crud, which can be even worse than just letting it be. They're also quite powerful solvents, which are really the last thing you want in your oil. When you drain it, you're not going to get 100% of the old oil out, so there's going to be some left in there.
ATF does little to nothing as an oil system cleaner.
The Magnum V8s aren't especially known for sludge, so I don't think I'd be real concerned about it. Like was mentioned earlier, save your money for some wax.
If you do feel you want to clean the engines internals, do it correctly by getting a bottle or two of Auto-RX, and following the application instructions to a "T". Go to
www.auto-rx.com to order it and read more on how and why it works.