One reson is some of the wires will not be long enough.
If you look at where the harness goes into the cab on the fire wall side, right in the middle of that plug is a bolt. if i remeber correctly its 3/8, unbolt that and the whole engine harness will come unpluged. Then swap in the V8 harness, you should get a V8 harness from an 84 or you might have to move the pin positions around in the plug. All the things you mentioned will be part of that harness.