I believe this is correct...
http://www.moddedmustangs.com/forums/2-3-mustangs/155123-springs-struts.html
Here is a short list of common Mustang spring rates:
-------------------front-----rear
'89 Mustang 2.3L ----365 ---160
88 Mustang 5.0LX ----430 ---200
'93 Mustang Cobra ---530 ---180
'93 Mustang Cobra R --750 ---260
----------------------------------------
Main differences in 2.3L to 5.0L suspension...
K-member is narrower (equal to 79-86 width)
http://forums.corral.net/forums/general-mustang-tech/992560-four-cylinder-kmember-wheel-track.html
Steering rack is 20:1 (3 turns lock to lock) as opposed to 15:1 (2.5 turns lock to lock) and it is noticeable while driving.
Front spindles and brakes are equal to 79-86 models. Front spindles are not compatible with shock/strut packages for 87-93 5.0L cars. Aftermarket control arms may or may not work as intended, so I would be careful here.
Front sway bar is smaller diameter.
There is no rear sway bar.
There is no provision for quad shocks on the 7.5 inch axle.
Now, depending on what the planned future is for the car, a lot of this can be overcome. Many people swap to 5-lug, so the spindle difference doesn't matter so much. Aftermarket springs should be paired with aftermarket shocks/struts, so that is a good time to do the 5-lug upgrade.
The wheel track can be corrected by using SN-95 Spindles and control arms. The rack can be swapped to a 15:1 sport rack with no changes. Upgrading to SN95 is recommended but more costly.