Go with the Eibachs, you won't regret it...
About a 1 1/4 inch drop. Looks great, just be very careful on speed bumps and crests of hills on the local streets, you WILL scrape the cats from time to time. Handling seems to be a bit better since lowered with the Eibachs too. You will need to get caster\camber plates if you want your tires to wear evenly, don't lower it without them. I use the BBK plates and they work fine.
You might want to consider the "steering rack" bushings (I think that's what you call them) also to remove the bump steer wou will get after lowering. Its not bad usually, but my car is a daily driver and I'm seriously considering a set as I'm getting tired of fighting the wheel on uneven roads...
If its done right, a lowered mustang looks pretty mean...
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"The Yellowjacket"
1998 Mustang GT
13.97@103
"Traction Impaired" or "Nitto Needy"
"They think they can make fuel from horse manure.... Now, I don't know if your car will be able to get 30 miles to the gallon, but it's sure gonna put a stop to siphoning."
- Billie Holliday