If you found a killer deal, it's likely a no brainer. However, you're ultimate goal with the car will determine whether it was the best way to go. Stock 6.1 vs. stock 6.4, the 6.4s are definately a better engine for the heavy late model cars, especially if you don't plan a lot of mods. However, the extra technology of the VVT and VIRs makes it a lot more difficult to "hot rod".
While I've seen Chrysler documentation stating the 6.1 block is beefed up over the 5.7 block, I've never seen anything like that saying the 6.4 block is better than the 6.1. Plus, the 6.4 timing setup is pretty wimpy (just like the '09 up 5.7) and there's been numerous problems with the 5.7s now that they have some mileage on them. A gal in our Challenger club had her chain break cruising down the freeway and take out the whole engine at 60,000 miles. Luckily it was covered under warranty.
The bottom line of my opinion is, if you'll be happy with the engine in relatively stock form, the 6.4 should be awesome in your Cuda. If you're going to want to hot rod it and build it up a bit, you should get a 6.1. P.S., if it's a super deal on the 6.4, you could get it and then work out a trade on a 6.1 and some cash.
