40 seconds is the point that your glycolytic system maxes out (maximum production of lactate). Clyde Hart has made a big deal of touching on this 40 second point for 400, but is it really that important if you are racing 25% of that distance?
I do know that SMTC used to have a 400m workout that was 3X400 with 3 min rest. AS you might guess, there were lots of people throwing up on the track on a warm day, so they shortened it to 3X350–and John Smith still uses that modified version.
It could be that Carl went to 350 because everybody else did.