no offense, but I don't your going to have the room to do as big as a setup as your thinking.
I have a 93 Sedan Deville and 2 MJ-18s from Mach5 has been a daunting challenge, granted I'm going ported.
I don't believe the Crown Vics have same size trunk I have, however pretty close.
Doing some rough estimating, you need at least 40" wide to do 2 18s, and thats cuttin it pretty thin. Not saying it can't be done, just take into account measurements before you start buying and have to squeeze 3 18s in a 10 cube box