port lengths?

I am building a box for 3 JL 12 w3's. It is ported because the sealed box I was running them in wasn't impressive. I have a soundstream rubicon class a 1000 watt amp. Each sub is getting about 325 watts rms. The factory plans out of the manual call for the dimensions to be 14.75 long x 14 tall x 22 deep, my box differs slightly. The port length is supposed to be 4 in. in diameter and 16.2 in. long. All well and good, but unless I cram the sub into the corner of the box I can't fit the port in the faceplate, and even doing that I don't have enough clearance between the sub and port to make me comfortable. I can't fit my port on the face like the manual says. The tech writer for that must have had a few. I can't put them up top because the box is not tall enough. I have given up on the manual and need your help. I don't know what would be a good tuning frequency for these subs (in the back of a 99 chevy tahoe). If you know one I can use the port calc and find the dimensions. I am looking for a deeper louder bass than the sealed box. I can fit a 2 in port in the faceplate. One more question, if I cut the diameter of the port in half-from 4 in. to 2 in. could I cut the length of the port in half-from 16.2 to 8.1- and still maintain the manual's specs? I feel like I just wrote a novel, Thanks

Have you thought about using a flexible port like what JL Audio offers? That would allow you to get your length. As far as the measurements, that is up to you on the frequency you are trying to achieve with you box. I have never really tried getting an exact frequency when building ported enclosures, I have always just used the good old trial and error method until I find the way it sounds best. I would start with something close with what the manufacurer says then adjust the length until it performs to your liking. As for the diameter of the port, 1-4inch port would be the same as 2-2inch ports so that would help you there. Hope this helps