Martin,
If you are talking about hooking-up a Bravo 2 to a Cummins 5.9 @ 315hp I would not advise it; the torque of that motor will make quick work of that drive's gears and cone clutch.
I know CMD/MerCruiser was offering a 5.9 rated at 250hp mated to a Bravo (the "X" or diesel rated version); not sure if that is stil the case. Also, some have had success mating certain Bravo's (diesel rated) to Yanmar 4LH and 6LP motors (horsepower ranges from 240hp to 315hp if memory serves correct) with better longevity due to their lower torque.
I know this does not answer the ratio question ((though I would suggest visiting the CMD/MerCruiser website and see if that 5.9/Bravo set-up is still advertised - it may list ratios) but hope it helps. Good luck.
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