I done a wall in a 91 Lincoln Town Car, had 4 15" Pyle PL1590BL`s and 2 Pyle Mono Blocks PLAD3300D`s, I built to basically prove a point that alot if it is install, I done a 150.0 on the TermLab Certified for DB Drag and a 151.1 in Iasca IDBL, so the Lincolns can get loud:), I know with some good equipment I could have been alot louder.
4 18's might fit I was thinking on them before I went to 15's, But you`ll have to sacrifice some port area.
Here is a pic of mine, not pretty but functional.
http://i34.photobucket.com/albums/d126/cra...00130.jpg
http://i34.photobucket.com/albums/d126/cra...00133.jpg
Hope this helps you