Pix 6.1 port forwarding

[Yahoo]

Hi all   Got stuck with a hard situation with my pix (6.1) on 515.  Before implementing the cisco pix firewall solution on my network, we used to run ipchains with port forwarding features to handle http requests.   Internally we have a single web server running NES with several instances on different ports. From outside, using ipchains, we used to forward http requests on multiple external IP' s on port 80 forwarded to the same same web server on different ports.   How can i achieve this using cisco pix ?   TIA Regards Mrao