Any idea what sort of device is used on the remote end with the overlapping network?
If you had all these configurations on a PIX or ASA I would say "Policy NAT" but unfortunately I am not that familiar with Router NAT configuration format. I might need to refresh my memory.
Easiest way for you would ofcourse be if the remote end has an actual firewall. Then they could do the NAT for the L2L VPN connection and you wouldnt have to make tricky NAT configurations on a router.