No, those are 2.25" offset inlet/ center outlet, which would only work if you were going to use dumps instead of tailpipes. I've used that style with dumps before, but with 2.5" inlet/outlet. 5.0 Mustangs use offset inlet AND offset outlet.
42443 = 2.25" offset inlet/outlet
42543 = 2.5" offset inlet/outlet
If you've got stock size (2.25") exhaust parts... h-pipe, flowtubes, tailpipes, then the 2.25" mufflers will be fine. But if you've got upgraded 2.5" tubing, use the 2.5".
Hope this helps.
If you use 2.5" mufflers with 2.25" tubing, what most shops are going to want to do is just slip the tubes into the mufflers and fill in the difference with weld. This will work, but it will be ugly looking. The alternative would be to remove the flowtubes from the H-pipe, remove the tailpipes, stretch the inlets of the flowtubes and tailpipes up to a 2.5" diameter, then re-install everything, line it all up, then weld it all. That's why I say, most shops are going to want to just throw the mufflers up there and fill in the cracks with weld. If you don't care how it looks, then go with that.... I mean, it's under the car right? Noone's going to see it.
2.5" mufflers with 2.25" tubing isn't as big a deal as having mufflers with the wrong inlets/outlets. If you try to put mufflers on a mustang with either a center inlet or center outlet, they'll have to take a piece of scrap tubing, angle it between the muffler and the tubing and fill a bunch of angled gaps with weld and that's really ugly... this is all IF you can find a shop that would even do it.
The best thing is to get the proper sized mufflers that match your tubing, get a nice tight slip-fit, then have minimal welding. Then you know it's nice and clean and that the same amount of flow is maintained throughout the whole system. I've experimented with a bunch of different exhaust setups because I have good friends who own a Meineke shop and it didn't cost me anything, except buying mufflers, etc and I know it feels better to do it right and know it all fits nicely and is a clean job. I've had some crazy stuff and ended up not liking it.
Hope this helps!