The only way to know is to test. I've seen guys get away with crowding a port to the back of a box, I personally have had far better results when I cut it down to leave a port width or more clearance. Crowding the port will make it behave unpredictably. It may be great or it may be awful.
The difference in anything less than 3hz tuning is inaudible, 5hz is about where you can tell you changed something from my experience.
If you have some flare ends you should find yourself some sono tube of the correct diameter, get you some gorilla tape, and test a few different lengths and see for yourself what works with your sub in your build in your box in your car with the music you like.