I am going to put under drive pulleys on my 89 gt. it is mostly stock. Do I need to change anything else or can I just ad the pulleys and not have to change anything else?
I have read nothing but bad things about this. Big sacrifice for a couple of HP. I wouldn't want to under drive my my Alternator. You could probably make the same power changing your stock rockers to a more efficient needle bearing roller style.
To answer your question: No, you do not need to change anything else. Just go with name brand pulleys. I put the BBK aluminum pulleys on about 10 years ago and never had a problem. The voltage gauge will read a little lower if at idle for quite some time, but goes right back up with a rev. I cannot give any kind of dyno numbers for it, but it did fell quite faster right after the install.
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.
If your balancer is old or original, IMO change it while you are doing the pulley's.
The way i see it, the balancer on a fox should fall under the mandatory maintenance, because by the time you know it's damaged it may have done irreversible damage.