Thanks, guys.
LBD, I do confess to hearing the theme to Mission Impossible in the back of my mind as I was starting to think about how to meet her requirements. :-) I'd have never finished this project without SketchUp. Not only did it let me iterate the design, but it also let me do manual interference checking. The rotate tool let me virtually tilt out the blocks to make sure they'd clear the spools below and virtually rotate the racks to make sure they'd really open.