I have that book too.  When JD made the decision to switch over to mulit cylinder engines back around 1949, they were getting their butts kicked by IH and Ford.  And here is the kicker.  There was internal conflict in the company about which way to proceed.  The engineers like Willard Nordenson and a few of the higher ups wanted to drop the 2 cylinder engines.  However, LA Rowland manager of tractor production wanted to keep them going as the 2 bangers were JD's biggest money maker.  So president Wiman compromised.  They would keep updating the 2 cylinders but be allowed to start designing tractors with more then 2 cylinders.  That decision was made back when they were in the 3rd spot.  But as the 50s progress they moved up to number 2 and for a few years matched IH or moved ahead with the 2 bangers.  They might have been outdated and somewhat crude but one thing JD did do correctly is predict farms would get bigger and horse power demand would go up.  IH had nothing to compete with the 720 horsepower wise and when they did match it with the 560 they scimped on the rear end.  And the 660 was just a 560 wound up tighter.