BlackBox routers over T1

I am trying to connect a remote office with small workgroup to our main facility via point-to-point T1 using 2 BlackBox LR120A WAN routers. The workgroup is set up with the usual 192.168.x.x/24 IP scheme and the main facility is 10.23.x.x/16. What should I use for the gateway entries? Something else in the private IP ranges? Do the subnets have to match.