Why don't you put your 2600 as your boarder-gw? 
interface Tunnel1
 ip address 192.108.195.70 255.255.255.252
 ip router isis
 ipv6 address 2001:440:1880:ABCD::2/64
 ipv6 enable
 ipv6 mtu 1420
 ipv6 router isis
 decnet cost 1
 isis metric 2
 tunnel source ATM0/1/0.1
 tunnel destination 192.36.147.120
!
interface ATM0/1/0
 mtu 4470
 no ip address
 ip virtual-reassembly in max-fragments 64 max-reassemblies 512
 load-interval 30
 atm restart timer 90
 no atm ilmi-keepalive
 dsl enable-training-log delay 0
 dsl power-cutback 6
 dsl noise-margin -2
 dsl gain-setting rx-offset 3
 dsl bitswap both
!
interface ATM0/1/0.1 point-to-point
 mtu 4470
 ip dhcp client broadcast-flag set
 ip address dhcp
 ip nat outside
 ip virtual-reassembly in
 atm route-bridged ip
 pvc 0/35
  protocol bridge
  encapsulation aal5snap
-P
  On 23/06/2016 1:47 PM, Cory Smelosky wrote:
  Found the actual problem!
 IPSec VTI VPNs means I can't do source-based NAT/firewall rules to redirect GRE from
the DECnet tunnel IPs to a separate box as is currently configured.
 
 
 So, does this mean that you're still off the air?
 
 Regards, Tim.