I measured the belt length using a piece of wrapping paper ribbon. It measured about 113" which is right about what the CAD model said. The ribbon actually worked.
. The tricky one to thread is the one with the motor. I may move that to the front. I am sure it will be easier with a real belt that has some stiffness.
Econobelt has a 103" and a 122". I think it will be easier to to move the lead screws together, so I will go with the 103" I ordered three belts.
I wonder how they make these belts. I can't imagine they have a mold for each one.