nathan,
You are referencing the wrong struct member to get device numbers. You should be referring to st_rdev and not st_dev.

From man stat:

Code:

dev_t    st_dev;      /* ID of device containing */
                           /* a directory entry for this file */
     dev_t    st_rdev;     /* ID of device */
                           /* This entry is defined only for */
                           /* char special or block special files */

Also, if you know the major and the minor device numbers, you can use the makedevice macro to create the device number. The makedevice macro is defined in /usr/include/sys/sysmacros.h. Maybe you could take a look at that file as well.