basically it all depends on how you use (abuse) the tranny. Not too sure about the autos, but a rebuilder once told me that once you cross the 450 hp threshold, you are in no mans land, and not too many reasonable priced manuals will hold together if abused. I know there are lots of people who will say I am crazy, that they are using BW T-10's, or M-21's and they do just fine, all I know is what I have been told. He said one of the newer autogear m-22's should be good up to maybe 550-650 hp, but abusing the tranny still can lead to parts failures. And I know the price tag on one of those m-22's may not be considered reasonable, like around $1500-$2000.
Seems like I remember reading in the summit catalog that the richmonds were rated around 400-450 hp, but you can check that out for yourself.
If you want OD, then maybe an auto is the way to go.
So........
the decision is yours, just have fun and good luck. I am sure others with more expertise will help you out here.
Just letting you know what I have been told.