Toe-in can be more accurately be defined in terms of angle rather than inches. Why? If you have a much larger tire diameter (such as in a 6400), 1/8" toe-in would not give you the same angle as would a much smaller diameter tire.
If the Book calls for 1/4", that would make the toe-in angle very close to that of a 3100 truck set at 1/8".
So why does the Book call for toe-in in inches? So a farmer can set the toe-in without expensive alignment equipment.
That's my wild guess on the matter anyway.


1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission