Go to
www.car-part.com. Look for salvage yards in your area that has a motor from a 2003+ Grand Marquis or Town Car (Panther). IMO you would be much smarter to buy a whole salvage motor than attempting any in vehicle repair. You could also look for a Mustang motor but expect to pay extra.
In order to use a whole Panther motor you are going to need the Mustang specific parts from the original motor.
Today's salvage yards generally do not keep motor parts (only whole motors). Likely it would be the same price to find the heads as what the whole motor would cost. I personally would prefer to perform a motor swap than a in car head swap. Not to mention that using the whole motor saves the cost of gaskets and the work associated with major engine work.
Another thing to consider. When the short block of the original motor was re-used aren't you relying upon it being in good condition without really performing any tests?
When the old motor is out, either sell the parts or use it for a different project. Because salvage yards don't sell motor parts there are plenty of people out there looking for parts. I had no problems selling parts from my old blown up 2000 GT.