Be sure you have a original 5/8-18 castle nut; they are hardened to take heavy touque. There should also be a thick hardened flat washer next to the drum, under the castle nut.
If the axel taper is worn you may have to install a shim, these are sold by most dealers.
Start out by torqueing the castle nut to 90 lbs, and tighten the nut just enought more to be able to insert the cotter key. If your drum come up against and rubs the backing plate then you probable need to shim the axel; by the way do not forget to install the axel key before installing the drum and starting the torqueing operation.
Ron