not that difficult to do, i'm considering this myself. the thing i'm worried about is power. most 5.0's and 5.8's don't have half the torque are 6 has. the easiest way is a carberated v8 to use fuel injection is lot more work you will need v8 harnesses and computors, sensors,ect... i'm looking into crate engines from jegs and summit.
good luck
joe