Guys I noticed the front suspension setup for the 05 Mustang is ditto to the BMW 3-series (E46), except for the strut attachment to the spindle is by two bolts while the E46 is drop in. But this design matches that of the E36.
So having established this and I have changed the springs on the struts on my ex-E46; it's pretty straight forward really. The easiest way to is to loosen the upper strut nut while the car is sitting . Jack up both the front wheels off the ground. Since the sway bar is linked to the strut housing you have to relieve pressure by jacking both wheels off the ground. Rest the chassis on jack stands. After loosening the sway bar links remove the two lower strut nuts attaching the strut to the spindle. Don't remove the bolts yet.
Place a jack under the l-arm or the point where the spindle meets the arm ( a scissor jack is best) & slowly compress or raise the l-arm to relieve presure on the loose strut nut. Go back to the engine bay and remove it completely. The advantage of this process is you are using the weight of the car to compress the spring, relieve pressure on the strut nut. Slowly lower the scissor jack and lo the strut will come off (with the hat still attached to the strut tower; alternatively you can also remove the strut tower nuts holding the upper hat).
After the strut is free off the spring, remove or slide out the two lower bolts. Remove the strut hat which should be attached to the tower. To install the new springs you may need a spring compressor. In my case I was using coilovers and i really needed the hat. If the new springs are shorter you may be able to get away by compressing the springs very little or pushing on it (I did by pushing on it on the coilovers) and turning the upper nut a few turns. you can reverse the procedure to recompress the strut (scissor jack method and after re attaching the lower strut bolts and sway bar links, reattaching strut hat to engine bay, taking car off jacks, retighten the strut nut from the engine bay. This is it. If you are good at suspension work it should take about 1 1/2 hour per side.
For the money you have to pay someone to get it installed you might as well get a coilover kit.
Lowering the rears is a piece of cake. You will have to remove the shocks (at chassi or axle) and/or sway bar links and let the axle drop (taking care to remove the rear parking cable link to the chassis). slide old spring out slide new spring in (accounting for the fact that perches stay the way they are.) JAck up axle and reattach the shocks/sway bar links.
However lowering the car does alter the suspension geometry. Up front because of the l-arm McPherson setup only the camber should decrease. Since the hat's attachment points on the strut tower is fixed or does not adjust, the only correction option is the use of camber plates. The caster is fixed and cannot be altered unless the strut hat points are modified to move fore-aft.
The rear is more complicated. At stock height the lower trailng arms will point slightly upward (when looking towards the front of the car). The arms should not point downwards as it will cause wheel hop (axle hop). After lowering and should the LCA's point downwards (when looking towards the front of the car), the forward force of the axle will also result in a upward motion of the axle (the lca mount point on the axle is now above the point on the chassis, hence the axle will tend to move upwards or away from the road)while the weight of the car will try to bring it down (wheel /axle hop). So this will have to be corrected (LCA relocation brackets maybe).
Next the panhard bar or the track bar prevents the side movment of the axle . At stock height the length of the panhard bar is s.t it centrally locates the axle. When the rear is lowered (axle moves up) the panhard rod will move up in an arc thereby pushing the axle to the left (when facing the front & assuming the panhard bar connects to the axle on LHS). Hence an adjustible Panhard bar is the best way to relocate the axle back to the centre.
The third link or the single upper control arm has the same effect as the LCA's. In stock height it's designed to control axle twist and also plant the axle. After lowering the angle of thrust might change as well.
I have learnt that recorrecting the rear suspension gemetry to near stock specs. after lowering by the use of relocation brackets, adjustible Panhard and adjustible LCA's (if reqd) is the major budget hogger).
EDIT: I just recalled that the third link or the little upper control arm controls torque twist for the axle( prevents the axle to twist up and also controls pinion angle). Ideally one would want 0 degress of pinion or -1 perhaps but when you lower the rear the pinion angle changes so one might have to get an adjutible UCA to reset the pinion angle.