What year Vic? I had a 2004 for 6 years, had a dual 15" box built for me by a semi local guy for a pair of DC subs, it was subs up, port back, 6 cubic feet net (after slot port displacement) tuned to 32hz with about 70 square inches of port....box was externally 40"x 20" x 20" and dropped in without any issues, and still had plenty of room.
Just wondering why your length and depth are so low? I know the crown Vics (pre 1998) had slightly smaller trunks though. Really with those subs you want more box volume, and port area if you can, like 7 cubic feet net for the pair.
Id fire subs and port up, have 3 friends that all had Crown Vics that did that and all were happy with the output. Trunk lid going to rattle but a little deadening and spray foam helps.
That will help you figure out how long to make your ports to get your tuning right.