Assuming you've accounted for the displacement of the port and 8 ft^3 is your net volume, the port needs to be about 13" long. It will vary an inch or two depending on it it's a slot port or not, but anywhere between 12-14" will be close enough as to not change tuning noticeably. If 8 ft^3 is the net volume before any displacement from the port, add about an inch to the port length
If you added another inch to the width (making it 17.75"x4"), you'd need to have a port length of about 17" for the same tuning. IMO it's worth getting that extra few square inches of port area by increasing the port length a bit. The swept volume of the x18 is huge and will excite the port pretty violently at higher levels, so you want as much port volume as you have room for. Redcon has a good point. Going to a 4" width will drop peak vent air velocity by over 30 ft/sec at just shy of xmax and you shouldn't' get any port compression or noise with that size.