If you over-rev a 401 by getting it to breathe better, have a big basket handy to take all the parts home. The big GMC's were designed for low-RPM torque, and with their inherent balance difficulties that required all sorts of special procedures during manufacture to keep them from shaking themselves to pieces, getting one overspeeded even a little is flirting with disaster.

I worked for a machine shop where we did a lot of big-six reconditioning, and most of them required incredibly accurate rebalancing after a bore job and piston replacement to have any chance at staying together.
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!