thanks!Most of the GT350 specs (i.e. spring rates, sway bar diameters, suspension pieces and part numbers, etc) can be found in various threads in this forum. Even a simple google search will get you a wealth of info and links to forum posts that have relevant topics.
Some parts are GT350 specific and not swappable to a standard Mustang, at least not easily or cheaply (like the aluminum spindles or rear control arms for example) but springs and sway bars do, and compared to aftermarket pieces are very reasonably priced. I purchased the GT350 bars for my own car, I just haven't gotten around to installing them yet. They aren't as thick as aftermarket bars, but still bigger than stock GT bars and for the price they're hard to beat unless you really need the increased roll stiffness of aftermarket bars for the track. For a DD like my car, those bars are all I need at 1/2 the price compared to aftermarket.
The springs don't offer much drop though, and one of the rear springs is wound backwards on the GT350, so if you're looking for 1" or more drop for your GT the aftermarket is the only way to go, but there are options that are pretty close to the GT350 spring rates so the handling and ride quality should be similar while still giving you more drop. Also, don't overlook the shocks when replacing springs. That's one area that IMHO should not be skimped on but quite often is. Get the best shock you can afford, but almost anything will be better than any factory shock, including the GT350 or Ford Performance parts.
Just do some research on what works and what can and can't swap to a GT, and what's practical for the performance goals you're after.