I found a long ball screw last week which gives me all the screws I need for this machine. However in thinking about it some more, I realized it might work better to change how the dual spindles are oriented and place the long ball screw differently. See the sketch below. The spindles are now placed next to each other in the x direction. This will let me make the gantry narrower which should improve stiffness and make it easier to package the ball screws into the mill.
Now my y axis screw will be 10" longer than it needs to be though. It already has the end blocks, so I don't really want to shorten it. I'm pretty sure I can package it as is to get my desired travel in the minimum size package, but I'm contemplating just giving myself an extra 10" of y travel. This would give me a working envelope of ~31"x34"x22" instead of 31"x24"x22".