How and when? "When" is easy--the "basic design concept... for the new V-8 engines... started in June, 1964."* "How" is not as important as "why". First Guess: A wise man said "if you have to ask "why", the answer is usually "money"." A forged crankshaft is more expensive than a cast crank; but the cast crank isn't as strong. So the main bearing journals have to be bigger to provide additional overlap with the rod journals to improve strength. Saving money on the crank means having a bigger bearing bore on the block/caps--which is easier when the block is designed from the beginning to accept the bigger crank bore. Similarly, a cast-iron camshaft is cheaper than a forged-steel camshaft; but since the Nailhead is not designed to intentionally spin the lifters, you can't just drop in a cheap iron camshaft. Again, money savings on the new engine since you can position the lifters at some amount of offset from the cam lobe to promote spin, giving longer life while using "inferior" (less expensive) components. Having produced the engine from '53 to '66--thirteen years--the tooling may have been nearing the end of it's useful life. If you have to invest in tooling--you might as well tool up for a "new" engine design. And if that new design can use less-expensive internal components, there's even more incentive. Buick claimed that the 425 Nailhead was at the "limitation of the basic dimensions as established for the original 322 cubic inch engine*". Going bigger--like the competition was--meant that a new block having larger bore centers or taller deck height was required. Buick chose to increase the deck height, and therefore allow a larger stroke while keeping the full-skirt pistons. Tailpipe emissions regulations may have dictated a move away from the "Fireball" combustion chamber. If not the chamber design itself leading to higher emissions, then the Buick-radical ("extended valve opening durations and excessive overlap to make up for the breathing deficiency*") camshaft surely would have. It may have been easier to "sell" customers on Buick when you can advertise a "brand-new" engine design featuring improved smoothness (because of the more-mild camshaft timing). Those are my best guesses; supplemented with excerpts (*) from the '67 SAE paper introducing the 400/430. I'm happy to learn more from others as to why Buick changed engine families in '67.