Thanks, again, dmizer. For anyone out there curious, here is the solution that worked for everything I needed, including allowing my servers to mount the NAS cifs shares this thread is about. It was relatively simple!
Set the Ubuntu servers inside the LAN to fixed IP addresses, but NOT the external IP addresses - instead set them to addresses within the private subnet ( but outside the DHCP range. THEN set the router to use 1-1 NAT to match the static external IP addresses up with the fixed internal addresses. Now I have the best of both worlds - internally the servers can access the same network resources as the other ubuntu workstations cvause they are within the subnet, but they still are dedicated servers to the outside world on a fixed IP. And on this SMC router, the port filtering still works, even when using the 1-1 NAT.
Hope this helps out someone else. Laissez les bon temps rouler!