Swap from a 4-speed to an Auto

I have a 71 340 Duster 4-speed that I'm considering swapping to an Auto w/od. My reasoning is so I can put lower gears in it (currently 3:23) and still be able to cruise down the interstate with lower rpms. Am I way off base? Also, if I did and put the console in, will I need to cut the tunnel more or will the current opening work?

Thanks in advance