rear brake drum removal

Trying to remove the rear brake drum. Split the chain case and removed the sprocket and circlip. Clamped the spline in the big vice and tried to remove the large domed nut on the drum side – but can’t budge it. Tried heat as well. Before I try very long breaker bar and/or a big hammer thought I would check if by any chance it is a right hand thread? Any other suggestions gratefully received. Though perhaps I could knock the spline through, but does not seem to be the case.

Cheers

Douglas