rear axle problem

["jb"]

I have a problem with my short axle on my 67 sr v, it seems the 20" shaft is fatter than the 26 1/2" shaft ....the brake drums fit perfect on the long shaft but i try both and even a sprare extra drum and none will fit likes about a 3/8 to 1/2 inch gap between back and the drum lip and the backing plate. I have taken out the key and tried it, taken off the shoes, taken of the backing plate. and still no luck , even lightly sanded the shaft to make their were no burrs or gunk, it's like the shaft is two fat for the drum...? question are their different drums right and left or maybe I have a bad shaft. I do not have a mike gauge but I know somebody who does if i need to get the shaft miked.