Spring perch location is the same - overall it is 1 1/2" wider (3/4" each side). Very easy to compensate with the backspacing on the rear wheels. If you find one - check on the cost to find rear brakes for it.
Stock brake parts are hard to find and expensive when you find them. The calipers have the parking brake mechanism built into them and the rotors are lefts and rights due to the directional venting. 1 of the rotors was easy to get at any auto parts store while the other rotor was discontinued - who could figure on that one. I found that front Granada calipers will fit the rear but you have to use the Versailles rear inside brake pad and the Granada front outside pad. Then again, you lose your parking brake. In the future, I'm thinking maybe using the circle track caliper plates that weld to the axle tube and accept the light weight metric GM calipers. That will get rid of all of the heavy stock cast iron parts.
Check into converting the brakes to late model Crown Vic or Exploder. Those bolt on some 8" and 9" rears, are FAR LIGHTER, keep the stock e-brake system, and the parts are available at any parts house when you need to service them. I don't know the ins and outs of putting them on a rear that already has discs, but for drum rears the Crown brakes go on 8" big bearing (5 lug) and 9" small bearing rears. Then the Exploder discs go on new style large bearing 9" rears (aka Torino bearing). Figure out which the Versillies rear is, and you make be able to bring that rear into the 21st century.
Another option for upgrading the Versailles is to go with this adapter from Currie Enterprises which allows you to install 2000 model Mustang Cobra disc brakes on it. This is the way I've chosen to go on my car. The adapters are installed, but I haven't bought the brake parts yet, other than the Lockar e-brake universal cable setup. Here's the link for the Currie kit. John B http://www.currieenterprises.com/CESTORE/productsRE.aspx?id=2373
You should probably remind folks that big brakes like this need larger rims to clear them. Stock 14" rims surely wouldn't fit, and though I don't know for sure, 15" rims are probably unlikely.
