Yeah, I noticed the 80mm depth on the modules’ page and thought it was weird. I haven’t measured the actual depths myself, but the half of the 6U Rackbrute without the bus board is the deepest the case gets, so I’m guessing that 75mm is the maximum depth.
The Rackbrute manual (and the module page on modular grid) seems like it has inconsistencies in general. Pay close attention to the Power Supply section of the manual. It seems like you need to distribute your power between the two rows of pin connectors on the board. This way you don’t overload the power supply.
Maybe someone with more Rackbrute experience can clarify?