It's always good to decide on how far you intend to go with your build before you start buying/making mods. There's a big difference between a street cruiser and a full-on resto-mod. But, there are certain basic mods that solve the "factory-installed" flaws inherent in 79-04 Mustangs.
Every input goes through the chassis. Stiffening up your chassis (especially a convertible) will allow you to get the most out of every other performance mod you make. Subframe connectors, strut, shock, and K-member braces will all help. I'm also a fan of the Stifflers "FIT" System; it can be installed in stages if you don't want to do it all at once. But, the more bracing you put into the car, the more you are going to feel every bump in the road.
Bushings are important, too. Your car is 34 years old, and so are your rubber bushings. Depending on your ultimate performance goal, new rubber bushings, poly bushings, or even Delrin bushings will help improve the handling. I would suggest going with forward offset A-arm bushings; they make a big difference.
Adjustable shocks and struts are key to making these cars handle well. They allow you to compensate for the other
suspension mods you make.
Along with the adjustable shocks and struts, a larger rear sway bar (25mm) will help you get the car neutral. All of these cars come with factory-installed understeer; getting them neutral makes a big difference.
The biggest mistakes to avoid are lowering the car too much, and installing stiff, non-adjustable shocks and struts.
You have a lot of options for brakes. I've used OEM 99-04 Cobra brakes on my 1992 GT autocross car for years and they work really well (with the right pads).
I cover many of these mods, and offer up
suspension tuning tips, on my YouTube channel. If you check out my "ThoroughbRED" playlist, you'll see all the mods I've made to my 1992 GT to get it to handle better. If you have any questions, please ask here or in the comments on YouTube.
Here's a link...