To determine if shoes are wrong or rite just set them in the drum one at a time. then check if the lining touches the drum surface top to bottom.9 and 10 inch radii are different. 10 inch in 9 inch drum will hit top and bottom and not center.
I don't think it's possible to put 10" shoes into a 9" drum, and if he had 9" shoes and 10" drums it'd be a whole different problem.
But you have the right idea. You should be able to set the shoes into the drum and they should "fit" the contour of the diameter perfectly, or very very nearly so.
