Try asking the 5.0 forum. They will know more of the 351W specific details.
However, I can tell you that to do it correctly, you'll need a lot more than just the parts from the crown vic. You'll want V8 mustang springs all around and the thicker V8 mustang sway bar to control body roll. You'll also want to upgrade to V8 mustang brakes and struts. This is all basic safety stuff. The 4cyl car wasn't designed to handle a heavy v8 or the power it has.
I'm assuming you'll run an AOD or T5 transmission. If the 2.3 has an automatic, you'll need a crossmember and driveshaft from a 5.0 mustang or manual 2.3. You'll also need an 8.8 rear end, because the mustangs 7.5" rear is quite a bit weaker. Maybe your crown vic has one, but I bet it is quite a bit too wide for a mustang.
As for running the 351W, won't it be too tall with the air filter attached to clear the mustang hood? You'll probably need a cowl hood.
There will also be other odds and ends you'll need for whatever tranny the mustang can crown vics have... like shifters and control linkages. You didn't state the transmissions though.
Anyway, ask in the 5.0 forum. We have obviously kept the 2.3...