I suggest scrapping the quad shocks, and getting some decent lower control arms (spherical bearing on one end, urethane on the other).
Doing it right the first time, and having a restricted budget is tough.
If you just use springs to lower the front, the control arms are no longer parallel with the ground... the lower you go, the more angled they get. This lowers the front roll center MUCH more than the centre of mass, so you actually get MORE roll... unless you use stiffer springs. A lot of applications have dropped spindles so that you maintain the factory geometry. Unfortunately, I only know of one aftermarket source for the mustang:
http://home1.gte.net/aje/Struts.htm
You could improve the front suspension geometry with a k-member, but that starts getting pricie. Griggs gets great reviews, Kenny Brown's recent design looks awesome, and Maximum motorsports is supposed to be releasing one too...
I consider the CC plates a MUST. Not only for alignment, but the added caster reduces the amount of camber you loose from body roll (this keeps the tire leaning into the corner to counter-act the sidewall flex, and maintain a better contact patch). The added caster & the spherical strut mount GREATLY improve the steering feel as well.
I've heard that the KYB adjustables aren't stiff enough for some... not sure where they fit in, but here's a general list of shocks (increasing in stiffness):
Stock
Cheap upgrade shocks like KYBs or Monroe or Sears specials
Koni Reds (at full stiff)
Tokico Premium
Bilsteins
Tokico Illumina 5 ways
Koni Yellows
Koni DA
Heavier springs require heavier shocks... then you just have to balance ride, performance & budget, and you've got the answer.
I've got the FMS-B springs with KYB's (essentially stock ride, with improved handling). If I put anyone in the back seat, it bottoms out on bumps.
If I did it again, I'd get the FMS-C springs and tokico premiums (let the ride suffer a little for much better handling). That's only $135 + $300, so it's still kinda budget.
I also recommend some stiffer lower control arm bushings for the front. You really feel the little bumps more, but it helps maintain alignment while you're cornering. With such crappy geometry, you really gotta maintain what you have! (and the stock rubber bushings REALLY flex). I got a front end kit from PST, and had the ball joints & tie rod ends done too (your's are probably getting worn). If you don't think you need the whole kit, check out steeda's offset lca bushings... that'll give you a little more caster, and better weight distribution.
Do you have subframe connectors? They're really a must... especially when you start firming up the suspension... the chassis will just start flexing more.
Oh, and a lower chassis brace too. They're cheap, and with the firmer bushings, the turn-in response is VERY nice.
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'89.5 5L LX, hatch, Medium Canyon Red, 155K Km (No winters), 5sp, 3.08, GT interior, a/c, Alpine CD, K/N, Crane PS91, Walbro 190 LPH, March Pulleys, Mr. Gasket 180 therm (HP balanced), 3 core rad, MarkVIII fan, Variflow controller, Polygraphite front bushings, FMS-5300B springs, 6 KYB's, 73mm Calipers, Maximum Motorsport 4pt chassis brace, Western Motorsport Racing's Sub-frame connectors. Re-ringed bottom end, conv. motor mounts, melling oil pump, FMS-C50 waterpump, GT40P heads (blended, polli