I believe most of it should be a straight sway, physically. You'll likely need electrical connections, fuel connections, and intake manifold (among others) changes to a 3rd and 4th Gen., but the Gen III and Gen IV are nearly identical on the exterior (except for things like cam, crank, and knock sensor changes.)
IMHO, this is the exactly reason I'm trying to wait for a top dog model, so I don't have to spend $1K on mods' that will void my warranty. Regardless, there isn't anything quite like the sound of a cammed V8 though.
"...What IS true: We anticipated that this would happen - we are never finished - and yes, Ford DOES deserve to win now and then. To think that GM can come out with a car to make ford throw in the towel is simply foolhardy..." - fbodfather