Port Distance vs Enclosure Displacement

I'm putting a Soundqubed (audioqubed at time of purchase) HDC3 18 in an Acura RSX. I overlooked a couple of things when designing the box and I'm trying to figure out the best way to make what I have work. I cut 3/4" MDF for a 3'x 2'x 16" box with 1 6" aero port. Realized port area would be way too low so now I'm trying to figure out the best way to implement 2 6" aero ports. I know port distance to a surface should be around the diameter of the port (not sure if this applies for more than 1 port) so I can either have: 5.77 cubic ft box with 2" distance from trunk or 5.51 cubic ft box with 6" distance from trunk (port sticking out of box for both). Which is more crucial? If you want to check my math, sub displacement = .23 cubic ft, using 6" pvc with OD = 6.625" that I will flare and http://www.psp-inc.com/tools2.html for port length. Both should yield 32 Hz - 33 Hz which seems to be as low as I can go with what I have. 22" long ports for 1st scenario, 23" long ports for second scenario; Soundqubed recommends 6 cubic ft for HDC3 18. Thanks for any input