99-04 convertibles, mach 1s, and bullitts have cheap 3/4 length bolt-on subframes. They suck and do no good. I highly recommend upgrading to Maximum Motorsports full-length weld-in subframes. It will make a world of difference in your car including eliminating a LOT of rattles/squeeks.
If you think the stock mustang suspension is too stiff- I dont know what to tell you. It's a sports car GT and is designed to handle exceptionally well. Your stock struts are probably just fine. I have 70k miles on mine and half of that is with 2" drop H&R supersport springs. My struts still function fine. OEM struts do tend to bleed pressure quickly with age, causing violent bottom-outs in the strut towers...though they rarely blow-out completely.
If you want a really soft ride, at the cost of performance handling, get some drag springs and foxbody struts/shocks. thats about as soft as its gonna get- but the car's gonna handle like a wet noodle, and present a hazard in wet rainy conditions.
Have you considered airbag suspension? It's not really the proper thing to do to a mustang, but may be what you're looking for in terms of ride quality.
Thanks for the in depth reply.
Yes, I don't want to completly ruin the handling so drag springs are out.
The welded in subframes sound interesting.
Is there an airbag suspension for the Mustang? Who makes it?
I don't mind losing a little of the handling to get a softer ride, but I don't want a wet noodle ride either...
Go to steeda.com, they got adjustable tokico shocks and struts, sport springs, screw it actually go to steed and get the g trac stage 1 and stage 2 suspension kit. Its got all you need.