Different mounting systems. 90 is off the gutters while 120 is roof mounted. If it is just the bare rack and mounted on bars then pretty much anything is possible with enough time and a large chainsaw.
My 150 build - http://www.pradopoint.com/showthread.php?27423-A-Random-approach-to-a-Bluestorm-150-GXL-D4D-automatic
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