If you plan on using a port of substantial length as you have mentioned (60") it is likely you would need to fold the port within your cabinet to achieve this length unless you cabinet is 60" in one direction of course. If you plan on taking the time and effort to create a slotted port I suggest rounding the corners within your slots to at least 3/4" radii to allow for minimum impedance of airflow.
This is the type of rounding I am referring to:
http://i184.photobucket.com/albums/x...erBuild017.jpg http://i184.photobucket.com/albums/x...erBuild016.jpg http://i184.photobucket.com/albums/x...erBuild010.jpg
Do note that any area where air would be sucked in or blown out has these rounded edges both in concave and convex manners depending on location.
With first port resonance at about 112Hz you could create potential issues, but with proper planning you shouldn't have any issues. With use of a sufficiently high order crossover you will be able to stop any leakage into this realm. With my now complete build I will be using a 4th order crossover at 80Hz to achieve this end and my first port resonance is at about 118Hz. Another step you could take to combat port resonance with a port of this size is taking dowels with 3/4" radius and placing them sparingly within the port*. This will increase rigidity slightly while having minimal effects on air flow.
*To give you an example if I were to have used dowels in my build pictured above there would be two pieces per fold.