10 in drums

Ok, I called NAPA about 10 in drums for the rear of my 65 Formula S. The ones I had simple will not fit so hopefully they were the wrong size. A local guy who restores Mopar told me the drums from a duster would work. I have the drums off and have checked and double checked everything. The wheel cylinders are new as are the pads. I have replaced the brake lines and the master cylinder. There has been no fluid added as everything is new. The emergency brake is disconnected just to see if that would be an issue.
Brings me back to the guy from NAPA. He suggested take off the front drums and see if they were the same as the rear. He thought they were the same even though the book has a different part number for the rear. Is this even possible? Has John from NAPA lost his mind or am I just drawing at straws to get the car off jack stands and out of the garage for a bath and allow me to start on something different?
Thank as always.