Comcast Cisco DPS 3939B with Cisco ASA 5505

Hi

We have received a Cisco DPS 3939B and some static IP's.

We want to migrate the ASA from the old non Comcast circuit to our new Comcast circuit, utilising one of the new static IP's.

The ASA will connect to the router and the LAN on the internal interface of the ASA.

The ASA will perform NAT and IPSEC L2L with another site.

There seems to be conflicting options on how to achieve this. Some say get the router converted to bridge mode and change the ASA so that it's external interface receives the public IP from Comcast.

What is the best/easiest way to get this working.

Thanks

SC