What are the major differences between A904 and A727?

A friends Father races an altered in NHRA Comp class. He's running a V6 making over 600 horse power and a 904 that handles it really well. It's a wicked combination and it looks like a great ride!
I under stand that the AA Hemi Super Stock guys use a 904 inside a 727 case when they wanna set records or feel they just HAVE to win an event. The trans will generally last a few passes at that horsepower level.