If you are serious about the 4.3/700R4 combo you really need to consider either putting the 4.70 gears in the rear case or at a minimum going 23-spline for added strength.  The 23-spline was originally for the Turbo model and I bet the 4.3 will have more power than that.  The additional cost is minimal when compared to the reduced chance of failure.

In my opinion, if you are planning to build for more power with an automatic, you would probably be OK without the 4.70 gears.  The automatic will allow you to crawl just fine with dual cases with stock gears in both and the stress on the drivetrain would be less.

I have heard that with an automatic, you can't stop the rig with gears lower than 100:1.  The brakes can't handle the torque.  This gives you an idea of the gearing you would have.

I would advise a dual ultimate and keeping the 22RE unless you are playing in deep mud where speed is the only option.

LOW GEARS ARE MUCH MORE IMPORTANT WITH STANDARD TRANSMISSIONS THAN THEY ARE WITH AUTOMATICS, ESPECIALLY IF YOU HAVE ADEQUATE HP.
 

Excellent points... yeah I was thinking about this this morning when I was preparing to make the New Years sale purchase... I obviously only want to do this once and want to go into a ~$1000 investment knowing that it will handle some changes in the future. In order to get the sale price, it looks like I'm stuck with the 21 spline input. This is a pain in the ass.