Everyone in here freaking out over the idea of an LSx in a Mustang cracks me up. If you didn't already have most of the parts to build a solid 408, I'd say go LSx, if for no other reason than to piss people off. Seriously... who the **** cares if he puts an LSx in there.... it's not your car, calm down.
EDIT- I change my vote to 2JZ-GTE. That would be a badass swap.
I always wanted to see some one take a 6 banger stanger and throw one of those in there. Wouldn't be lying if people asked u if it was 6!