According to my manual when I looked, you could not space the tires with the duals to go down 30" rows.  My guess is that there is not enough clearance when you turn to have the duals spaced out with the inner tires set for 30" rows (obviously you're not going to get them down to 60" centers).  I'm also all but positive that you can't put 38" rubber on them for the same reason, due to the crab steering, the tires would hit (I'll have to look in the manual, but I'm pretty sure about this, too.).  Finally, you could order the tractors with either 24.5-32 rubber or 30.5-32 rubber, rather than the 20.8-34's or the 18.4-34's, but I don't know how hard it would be to find the right rims if the one you have didn't come that way.  The larger single tires listed can definitely be spaced for 30" rows, per the manual.

Mike