Problem is neither wheel is 4 lug.
The Impala wheels are 5x115 bolt pattern
The Ford Crown Vic wheels are 5x114.3(5x4.5")
The sweetness would be(if you had 5 lug swap) the Ford Crown Vic wheels with the chrome center caps. The Ford wheels have a center bore of 70.50mm, so they'll easily fit over the smaller 66.06mm Nissan hub.
The best part is you can get the Crown Vic wheels in 16" 5x114.3
The caps come in a couple different styles, from different years. There's a flush-style cap, that barely protrudes, then there's a more bulky cap.
THIS IS THE LINK YOU NEED TO BUY THE HUBCAPS:
http://www.hubcaphaven.com/police_88_02.htm