can anybody explain ports?

This is a broad topic so I'll post a link to a former discussion on this forum on this subject to get you started: topic: why are ports louder?  That thread has some great information.

Here are two basic ways to build a vented box:  use the recommendation that the manufacturer publishes exactly as shown, or use a design software program and generate your own model.  I typically use Unibox for this.  WinISD Pro is another good free program.  They are included in the list of links below along with some various other help tools and sources of info that I've saved to file:

https://www.diysubwoofers.org/faq.htm
https://home20.inet.tele.dk/kou/ubmodel.html
https://www.lalena.com/audio/basic/terms/
https://www.hometheaterhifi.com/volume_5_2/cmilleressayporting.html
https://www.carstereo.com/help2/Articles.cfm
https://www.clubknowledge.com/Car_Audio_FAQ/?t29
https://www.linearteam.dk/default.aspx?pageid=winisdpro
https://www.mhsoft.nl/spk_calc.asp#top

It boils down to this.  In order to successfully design your vented enclosure, you have to put the time and energy into learning as much about the subject as you might learn about a subject in a typical school course.  There can't be TOO much information, but there are sources that are more easy to understand than others.  A design program like I mentioned uses the complicated math behind the scenes, and leaves you responsible for using the software correctly.  I like this way best because complicated math hurts my head. 

Repetition builds excellence.  I would recommend that you start by downloading WinISD Pro and open up the Help file.  Pick a woofer in its database and click through the dialogue boxes, seeing what the program does to automatically design a vented box for that woofer.  Meanwhile, refer to Help to understand each step of the process.  Eventually you will be able to input your selected subwoofer into the program and start working on it.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

In plain English:  Ports work by making sound from air pressure moving across them.  Have you ever blown across the top of a bottle?  Hear it make a sound?  That's how ports work: the speakers moves air that blows across them.  Now go back to the bottle and put some liquid in it.  Blow again.  Change the amount of liquid.  Blow.  Notice that the sound changes depending on how much liquid is in the bottle?  Try a bottle with a larger or smaller size opening.  Notice it makes a different sound?  That's the same thing that happens with different size ports or different size enclosures.  It's called a Helmholtz resonator, if you feel like another Google.

A ported anclosure is much more difficult to design and build and make sound "right" than is a sealed enclosure, probably by a factor of ten.

It would be close.  The tuning frequency spec'd here is 32 Hz.  Increasing box size and number of subs as you stated, while increasing port opening to 3" and leaving the rest of the port dimensions the same is very close to correct.  The tuning frequency may change by one or two Hz.

You could probably divide the box so that a pair of subs on each side share a chamber and a port.  If you make each side 3 cu ft., the dimensions for each port would be 3 X 12.5 X 27.5 to maintain the specified tuning frequency. A 29" length would have still been very close, less than 1 Hz off.  And keep in mind that the port structure, in its entirety, is infringing on the air space for the subs.  So it must be calculated as additional volume on top of the 1.5 cu ft needed for each sub, as well as bracing and driver displacement (unless the JL specs tell you that the driver has been included in that given spec.)

(I checked this out using WinISD Pro with a random subwoofer to come to this conclusion.  That's a good reason to spend enough time getting familiar with the program, so you can look at how to change port dimensions around...even if you're using very close to the manufacturer's spec'd box and port sizes.  The program allows you to switch from round to slot ports, so it's faster and simpler than having to use pi R squared calculations for conversions.)

charlie, it would be worth looking into, IMO.  That box is stated as having .7 cu ft but you'll want to measure it to be precise.  And the Rockford link tells us the sub will use a .625 cu ft volume for a good response.  Probably the right amount of volume difference to put in a port.  The parameters of that sub call for a ported box as well.  Work out the port size for a .625 ft^3 box and see if the port volume would make up the difference in box total volume.

And check the sag factor for a down-firing woofer with this sag calculator.