I asked the guy wo has been grinding my crankshafts for the past 35 years that same question a few weeks ago, and got the same answer- - - - -inline sixes don't need the crankshaft balanced at any normal operating RPM. The 120 degree separation of the counterweights creates an inherently stable condition. Recently, I had a 235 crankshaft in my lathe, and turned about 1/2" off the OD of all the counterweights to create clearance for a shortened connecting rod and piston combination I'm using with an increased-stroke engine I'm building. Just for grins, I turned the spindle RPM up as fast as it would go. No shake! A V8 crankshaft would have made the lathe walk across the floor without properly calculated bobweights on it!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!