Center Port for Zcon

[Dexter Chaos 14]

So I am designing a new enclosure and this one will have a center port. I have concerns with my port width when it reaches the rear of the enclosure.

This is for 2-Zcon 12's,SAZ3500 power,currently box will be at a net volume roughly 7.8 cubes tuned at 32

 

the distance from the port wall to the flare is 4.33 inches if you cant read the photo

 

Since it is in the center do I need to maintain the same width(in my case 6.5inches)? 

 

Is there a rule of thumb for this?

 

Here is a pic of what I currently have designed.

 

 

[gss 11]

That seems like a very big box for 12's.

3.9cu.ft. per is waaay more than recommended, is there a reason for going this big?

[Dexter Chaos 14]

That seems like a very big box for 12's.

3.9cu.ft. per is waaay more than recommended, is there a reason for going this big?

So that I can shrink it if nesasary.

Anyone help me with my port?