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
