In your scenario, I would install a 4-port HWIC Ethernet card into both WAN routers then have your WAAS box plugged into them. One interface configured as the primary and the second one configured for standby. You would need to have an Ethernet cable plugged between the 2 WAN routers for transit traffic. Configure L2/forward and GRE for the return traffic and use WCCP negotiate return for the egress method. This is how all of our remote sites are setup. A second method is if you don't want to plug directly into the WAN routers, you can replace your l3 (model ?) switches with a stack of 3750Es and make them look like one logical switch.