Astaro Gateway between ISP and ASA

I want to put my Astarto Gateway between my ISP's router and our ASA5510. I want to keep the 5510 as the Firewall and use the Gateway for Intrusion Prevention and mess around with some other stuff.. I have half a dozed IP's. My guess is I give the ASG's WAN one of the unused IP's. What would I give the LAN side... ?? Would it be kind of a "pass through" by "cloning" the ASA's MAC...??

Thanks for any help

:D