Will it fit, yes, but not without alot of modifications i'm sure. You have to remember that the 5.0L has a totally different oil pan design, so getting a 5.0L to fit where a 3.8L or 4.6L once sat isn't going to be just a bolt in. It may be that the entire front crossmember (for lack of a better term) needs to be replace to fit the 5.0L in there. Then again you'll need the tranny, probably a driveshaft, and i believe the rear end in the 3.8L is smaller so you'll need to replace it with one from a 4.6L.
Then even if you get the 5.0L to fit you'll need all of the electronics from the 5.0L car or you'll have to adapt everything from a 4.6L to fit. The electronics, and fuel injection system from your 3.8L wont work with the 5.0 or 4.6.
Trust me, you'll be much happier going with the other route. Sell your car, and buy something with a V8 already in it.