Good deal, NMO27's were always good antennas for me. We were always using full size trucks, so mounting them top dead center really helped. They performed very well and were easy to tune. Nice low profile antenna, at least for a CB antenna, and definitely looks more professional that some of the consumer grade CB antennas out there. I doubt you will have any issues with a trunk mount. About the only issue I see is the whip contacting the roof when you open the trunk.
Not suggesting you and your wife were playing cop, only pointing out that they look good on a Crown Vic, if done right. In my own personal opinion, a crown vic, of any flavor, just doesn't look right without at least one antenna on it. I bet that is one nice road trip car. Lots of room to stretch out and a huge trunk. One of my co-workers has a couple of kids and fantasizes about owning a Crown Vic and using it for road trips. Not sure if he wants the cage between him and the kids or not. Sounds like it might be useful in some instances.
You can get the NMO27 in a grey coil/chrome whip. black coil/black whip or black coil/chrome whip option. If you find a good shop, they will sell the coil and whip separately. The black whips look good for a while, but the black coating doesn't last too long, especially if it contacts garage doors, roof, etc.