What 6 speed, or even 5 speed, auto transmissions have you heard of being swapped into a 2nd gen gasser?
in a perfect world I would build my 408, have 4.88 locker rear, and a 6 speed with double overdrive in between.

 

It's my understanding that the bellhousing bolt pattern on the newer transmissions is different than the magnum v-8s, you could likely make an adapter, but, then you would still need a controller for the trans..... There is one company that makes such things for the diesel trucks....... but, you are looking at a couple grand for JUST the controller, and more money for the adapters, and a fat chunk of change for the trans.

For the investment involved, you could buy an awful lot of gas....... You would never get a return on the investment.