So I have been planning on upgrading my suspension on my 2003 convertible stang, what's the best adjustable system kit I can buy, not ordered yet because I have heard that the convertible require a larger load capacity, open to all suggestions.
One of my autocross cars is a 2000 GT convertible. Over the years, I've found it can be better (and less expensive) to piece together the suspension components you need (or want) than to buy a kit. Is this a street car? Autocross car? Track car? GT, V6, or Cobra? Do you want coil overs, or an improved stock-style setup? Knowing the answers to those questions will help us provide suggestions for mods.
Some companies make convertible-specific springs, but some don't. For example, the Ford Performance C springs work on both coupes and convertibles. I used them on my 1992 GT, and my 2000 GT convertible (before I swapped in an IRS). Our cars handle better when they sit higher (especially if you're not using coil overs), so I would not recommend lowering the car more than 1". Low Mustangs look cool, but limiting suspension travel makes them handle worse.
One mod that's extremely important when it comes to making a convertible handle better is chassis bracing. A good set of full-length weld-in subframe connectors should be on your list. Every input you make, steering, accelerating, and braking, goes through the chassis. If this is a performance driving build, I'd suggest going with the Stifflers FIT System. I use it on my 2000 GT convertible, and it makes a huge difference in how the car handles.
Getting these cars to handle better is all about fixing the "factory-installed flaws" (understeer/throttle-induced oversteer). Adding adjustability is the right idea. Adjustable shocks & struts make a huge difference. The Tokicos I use have been discontinued, but most people I know go with Koni Yellow single adjustable shocks & struts.
Installing a larger rear sway bar (Eibach 25mm) will help reduce understeer too.
I would also recommend inspecting and replacing all of your bushings. Every bushing on your car is 18 years old (and probably not in great shape). For a street car, rubber is fine. For a performance build, you can look at poly or Delrin in certain areas. Using offset A-arm bushings will help too. I've been very impressed with the Global West Del-A-Lum bushings, but Steeda offers a poly version that would be fine for the street.
If you plan on changing your rear upper & lower control arms, use a set with 3-piece poly bushings.
If this is a performance driving build, consider spherical upper differential housing bushings (sometimes called bearings). J&M makes a good set.
An important part of upgrading your suspension is knowing how the parts you choose and the adjustments you make will impact the handling of your car. I made this video to help people adjust their autocross suspension. It's about my Fox build, but most of the info applies to your car too.
This is another video from my channel about the common mods people make to their Mustangs that make them worse for autocross. The parts of the video that cover suspension mods should help you.
There are also videos on my channel that cover all the chassis and suspension mods I've made to my cars. If you have any questions, please ask here or in the comments on YouTube. I'm happy to help.
Thank you for the tips, I was planning on building it as a show car and doing a 5.0 coyote swap later on but leave all my stock badges, it's a 2003 convertible v6. Saw it sit to long and she needed some love, I was wanting to go with adjustable to give it a stance at shows, and adjust the ride height as I need to, I couldn't see lowering a mustang to 1" and expecting it to look good, they are true ponies and should have a muscle car stance, still throwing ideas around and not sure, I'm trying to get the best kit or build my own as you said, and definitely bracing the chassis, all around want a smooth ride with the ability to lower or raise it with the turn of a wrench
You are not going to find better advice than what @Warhorse Racing has provided. I upgraded my 2K GT convertible along the same lines, but non-adjustable...frame connectors, shock tower connectors, Koni STRs, bigger sway bars, HD rlca and uca. I kept my stock springs bc the roads I frequent have some nasty dips and RR tracks. Handles like a go-cart. Happy hunting.
That's my problem I am having, the roads around Montana are not the greatest, and there's alot of RR tracks around here as well, imma just build it by pieces and keep my shocks