How much money are you investing into this project? What amplifier are you running? What is your current electrical? (batteries, alt, etc) I just want to see what kind of rms you'll be sending your subwoofer.
Based on these numbers you could probably get a away with running a slightly smaller box. Some members may tell you that 6 cubes would be easy to make, but in my three boxes, I could only come up with about 4.5-5ish after subtracting sub and port displacement. The other thing with our trunks is that you will not be able to get an 18" on the vertical face of the box, you will be running the sub facing up. As far as trying to meet box specs, I don't want to stray you away, but possibly more power on a 15" would get you what you need. I run a 15" DC Audio Level 4Xl on around 3k. She does well.
As you can see, even she is pushing it for height. By the time you build the box high enough to barely get the 18" facing back in a 5-6ft^3 box, you'll be "choking" the airflow off from your cabin. If you know what I mean. But running the 18" facing up, expect some SERIOUS trunk rattle. lol A 15" facing the rear would do you just fine.