IMO the price of salvage yard 4.6's is sooooo low in general that you should NOT get hung up on this particular 4.6 over the rest of the "fish in the sea". You are correct about the 4.6 being an interference motor. I wouldn't touch any 4.6 with timing set damage for that reason.
Check out car-part.com for examples of other 4.6's in your area. Don't forget to search on Crown Vic, Town Cars, and Grand Marquis.
IMO you should focus first on what the over all "plan" for the project is. It's not as easy as you might think to put a modular motor in a car that did not originally come with one. The decision should depended on factors such as "your budget", "your plans", and "YOUR SKILL SET".
Read the following thread: http://www.allfordmustangs.com/forum...ere-first.html
You may find doing an old school OHV 5.0 is easier.
I'm not an expert on exactly which model years are best to use for the 4-5 lug conversion. However, I don't think that the 1999+ are it. Something about the width of the rear end. But maybe someone with more specific knowledge will chime in.