IIRC,
The 250 has 2" blocks, the 350 has 3"
you can get the same spring pack on the 250 as the 350 (ie...my 250 has the overload spring)
the front and rear axles are the same in the 250 and 350, except for the rear axle in a dually 350, obviously. starting 2002 they went to a "true"...