No need to change the harmonic dampner, the pullies on a 5.0L OHV are just bolt on. If you're still running the stock alternator, I might think twice about it. Even with the stock pullies, the stock 75A unit is barely able to keep pace with the current electrical system. With underdrives, it struggles. Even more so if you've got an additional electrical load like a big stereo, or electric fan.
I've got them on my car, but I'm also running a 150A 3G alternator with the smallest alternator pulley available. And I still get a dip in the dummy gauge when there's a big draw.