You DON'T. MASQ is a 1:Many NAT setup, which is the incorrect tool to perform what you are looking for. You are looking for a Many:Many NAT solution, which is the traditional NAT setup. Take a look at Section 7.28 FAQ entry below for more details on the IPROUTE2 tool which would perform what you need.
For users out there who are thinking about enabling multiple IP addresses on one internal NIC using "IP Alias" and then PORTFWed ALL of those ports (0-65535) and use IPROUTE2 to maintain the proper source/destination IP pairs, this has been done SUCCESSFULLY on 2.0.x kernels and less successfully on 2.2.x kernels. Regardless of success, that isn't the proper way to do it and it is not a supported MASQ configuration. Please, give IPROUTE2 a look.. its the right way to do a true NAT.
One thing to also note:
If you have a bridged DSL or Cablemodem connection (not PPPoE), things are a little more difficult because your setup isn't routed. No worries though, check out the "Bridge+Firewall, Linux Bridge+Firewall Mini-HOWTO" on the LDP. It will teach you how to get your Linux box to support multiple IP addresses on a single interface!