When you take the centre hub nut off, does it have that curious sprung steel washer wrapped around it?
If so, its possible to release the entire hub by undoing the 4 bolts on the backplate.
Does the drum move outwards a fraction then binds? If so, a rust lip is holding it there, but if it doesn't pull out at all then the bearings are rusted to the stub axle.
No matter.....try revolving the drum and giving it a healthy clout at the same time (copper hammer preferred).
If still no joy, try prising the drum away from the backplate, gradually and a bit of a time. (The shoes will probably be sacrificed here).
If all else fails, then its slide hammer time which will pull the drum away, but will probably wreck the shoes, hold back springs and maybe the dust seal from the slave cylinder.
We've all been there to some degree......
John