The big 6's were very good at what they were designed to do- - - - -deliver stump-pulling torque at low to medium RPM for a long time. Hotrodding one would be an exercise in frustration. For the same reasons, you won't see any Clydesdales or mules running in the Kentucky Derby.

I worked at an automotive machine shop where we tried to keep some of the older big sixes rebuilt for a few companies with GMC medium-duty trucks still in service. Any attempt to mix & match parts, particularly pistons, rods, and crankshafts, was a disaster. Apparently, there was a lot of custom balancing done at the factory, and even reboring an engine necessitated very precise piston weight matching to keep the engine from shaking itself apart. High RPM would be the death of one of those engines in very short order.
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!