Buy a good running pullout from a 96-99 Explorer, swap out the cam with a good GT cam, swap the springs with some new TFS springs, drop it in and run that.
$2k is enough for a basic rebuild and the google will tell you that pretty much any motor can go into a mustang.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.