and what's a good break in for the motor might not be a good break in for the clutch/rearend. A hard break in is also certainly not good for lifters.
as IJ said, there's alot of variables, and alot of parts that could come apart if it wasn't thought of. A gentle break in is the safe way to do it, but for the most part... if she's gonna fly apart under load, she'll fly apart under load, now or later. Would you rather know now? or be waiting on a timebomb?