generally you need about 12sq" of port area for every cu'...so in your case you looking at about 55sq" of port. if you went with precision ports (flared) you could do like 3 4" ports.
use the formula in my sig to find the length for you desired tuning.
if your going to use round ports, divide you box by the number of ports your going to use and plug the numbers in. this length will be for each port.
if your going to use precision ports, do the same as i said above but subtract an additional 5" and cut you center piece to this length.