In that case you probably want to do NAT. You don't want to configure a whole lot of cluster IPs. You will only configure one cluster IP, and tell your upstream router to route to the firewall cluster IP for the range of addresses you will be using for NAT.
You can do this with either routing - use something other than your NAT range for the network between the firewall and router - or you can do it with proxy ARP.
Read through this post
http://www.cpug.org/forums/showpost....7&postcount=17 for some stuff about routing and NAT.
I think you've got a bit of reading on routing, NAT, and clustering coming up.