All I can tell you is what I have been told, as I am no engine expert, but... 41 is about the highest you want to go. And at 41, you will have good power initially, but once the engine gets hot, you will lose HP quickly.
As for Red Box V Black Box, everyone has an opinion. I have always liked the Red Box, as it is simple, has a good rev limit, and allows the use of the blue plate for adjusting the timing.
The black box works well also, but you can't manually adjust the ignition timing like you can with the red box.