You'd have to find a Mustang shop that knows what you are talking about. You need a complete IRS, you need the pinion flange from the DS and you need a IRS catback. If you do find a shop that will do it prepare to lube up. Figure $1200-$1500 shipped from ebay or a forum member for the IRS, $50 for the flange, $400 for a cheaper catback and then whatever the labor rate is if you can find a shop to do it. Then there is the fact that you'll get massive wheel hop and deflection with the stock rubber bushings. Another $700 to get the FTBR Delrin kit and the labor to put that in too before you bolt the IRS into place.
I'm not trying to dissuade you from this swap, I'm just letting you know what you are in for $$ wise. And like I said you'll have to find a Mustang or speed shop that will even do this.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.