There are other threads about this, search around. I've replied about this a few times, see
https://www.sn95forums.com/threads/...-unit-over-the-stock-radio.88609/post-1584040 for one. Figuring out which Metra adapters to use was half the problem, and I posted notes about them in some of the linked threads.
The Mach system wasn't bad when it was new, but Ford chose to use unusual ohm speakers, so you can't replace them with newer aftermarket ones. I was never able to find a source for the correct speakers anywhere other than used on ebay or in a junkyard, which wasn't a great option. I was also surprised to find that the way the speakers are wired is not what you might expect. The four woofers are wired in pairs, each amp does two of them. The frequency crossovers are also not what you'd expect if you're used to a typical aftermarket system, so aftermarket speakers connected to the OEM Mach amps don't get the same benefits as you'd think. You can replace the head unit with an aftermarket one but IIRC you may have to adapt the wiring to prevent a "pop" from the Mach amps.
After going down the same decision tree as you for my 97 GT convertible, I went with an aftermarket double DIN radio (I used a New Edge era radio bezel and the associated sub-dash surgery to make that work), a small 4 channel amp (100W RMS x 4 channels @ 4 ohm) mounted where one of the original rear amps was, the crossovers mounted where the other rear amp was, aftermarket speakers in the stock locations (Kicker 47KSS6804 front and 47KSS6504 rear), and a small powered sub (100W) in the trunk. It works well, the crazy amount of Dynamat I added certainly helps too.
The speakers were pretty easy to mount in the stock locations. The woofers are standard sizes and just mount directly. The tweeters required a custom bracket bent from a piece of sheet metal I had laying around. The brackets were not hard to make, I just bent it up in a vise by hammering some right angles into it so the tweeter sat sufficiently recessed in the original housings. I spray painted them black and they disappear behind the grills.
The hardest part is running the wires from the head unit to the amps and then out to the crossovers and then to the speakers. I needed to replace my carpet anyway, so it was a good chance to pull everything out to do the work. This also allowed me to inspect the floorpan and correct any issues - convertibles tend to get wet inside eventually, and that settles in the carpet/padding, making a perfect breeding ground for the dreaded tin worm to eat the floor pan. Thankfully I only had some surface rust in a few places, and was able to quickly banish the early stages of tin worm and coat my floor pan in some fresh paint. Then it was on to Dynamat, wiring everything up, installing the new carpet, and putting it all back together.
I wanted a stereo that I could hear on the freeway with the top down, because I enjoy tunes while I drive. I don't need something loud enough to blow my eardrums out - younger me already did his best attempt at that already by rocking out way too hard and way too often, unfortunately, mostly while driving. Selecting quality but reasonably priced equipment achieved a very good result without breaking the bank. It won't win any stereo contests, but it sounds good, has Android Auto/GPS, and I could easily install a backup camera while I was in there doing the rest of the wiring work. Honestly, the Dynamat was the most expensive individual thing I bought (3 bulk boxes, yeah...), and that was because i went kind of nuts with it. The car feels incredibly solid now (the doors close with a very satisfying thud), but the added weight of the sound deadener spread all over the car and the heavier carpet I picked is noticeable, though not in a bad way. If I was reusing my original carpet and went with a more realistic amount of sound deadener, the cost and effort required to install it would have been much less.
