Cisco Firewall :: ASA 8.3 Dynamic Policy NAT

I have devices on Inside interface of ASA that need to get to Internet to get ntp. Hence I want to set up dynamic pat (interface overload) which 8.3 style would be
 
-object network obj_NTP-DEV
-host 192.168.1.250
-nat (INSIDE,INTERNET) dynamic interface
 
But I need to limit nat to only Internet destined traffic on ntp port not all ports for traffic from 192.168.1.250.I'm not using this nat set up to control outbound access - I also have incoming RA VPN tunnels to the box and traffic from these sources need to be able to get to 192.168.1.250 and the above simple set up would break that access as all traffic involving 192.168.1.250 would get nat'd
 
Reading the doco I've sent myself round in a loops trying to figure how you are meant to do such a  " Dynamic Policy NAT (overload)" call it what you will config in 8.3