hap - Thanks for posting your question on the forum!
I just want to clarify that based on the firmware you mentioned, it is actually a 2nd Gen 3200. This means you have the ability to use the web interface to configure this device where in the 1st Gen 3200, this option is not available.
You have 3 options to configure this as a pass through:
1. . The easiest option may be just to configure IP unnumbered on the WAN interface. In this option, you would assign on of your usable IP addresses to the ETH 0/1 interface of the NetVanta. Then, under the IP Settings of the WAN interface, you would specify "IP unnumbered" and select the ETH 0/1 interface. The rest of the IP addresses can be used by the firewall with its gateway pointing to the NetVanta.
2. You can request that your T1 provider give you a WAN serial IP address in addition to the LAN block you already have. You would assign the serial IP to the WAN interface of the NetVanta and assign one of the LAN IPs to the ETH 0/1 interface. The rest of the LAN IPs can be used by the firewall which will point to the NetVanta ETH 0/1 IP as its default gateway.
3. You can split the LAN block you already have into smaller subnets. However, you will lose usable IP addresses in doing this. You would assign one half of the subnet to the WAN and the other half to the LAN interface of the NetVanta.
You may find the following document helpful. It does not match your application completely, but it will help in configuring the WAN portion if you have not already done so: Configuring T1 and E1 Internet Access in AOS
Please do not hesitate to let us know if you have any further questions.