It has been a long time since I have done NATing on an IOS router 2600 series with 12.x code.
Assume that f0/1 is IP NAT inside and f0/0 is IP NAT outside.
There is a VPN tunnel with 188.8.131.52 from the outside connecting to my 10.10.10.10 on inside.
When 184.108.40.206 from outside hits my 2800 router to access destination 10.10.10.10, I want it to translate to a source of 220.127.116.11.
It's a piece of cake to do it on an ASA.