I've never weld so I can not say for sure but I am pretty sure that is too rusted to weld right onto. If you can cut off back a bit to where there is just surface rust, clean that off and possibly take it to Meineke and have them weld a new pipe onto.
You can also try going to a Ford Dealership to get the part number for the exhaust and then try and find it again from there for cheaper somewhere else. Another good spot to start off would be MOTORCRAFT® PARTS FOR FORD, LINCOLN OR MERCURY VEHICLES
or more specifically www.MotorcraftService.com
which sometimes have shop manuals, wiring diagrams and exploded parts view that may help.
Also check out some of our site sponsors Mustang Parts & Performance - American Muscle
and Anderson Ford Motorsport Home Page
, they might have some goodies to pick up if you'd like to do an upgrade to your exhaust. Older site sponsors, that is some other places you can check out are Summit Racing or Atomic Performance.
What a lot of members may recommend if you want a better exhaust tone and a bit
more performance is to get the exhaust off of a GT (a.k.a. GT Takeoffs) and have them welded on at the mid pipe. Any exhaust from 94-94 should be fine, 99-00 should work well too. I say should be fine because the tailpipes turn down at the end, on 01+ the rear bumper has exhaust cutouts.
Good luck with all that rust