building central beam staircase

We want a central beam staircase in our new home and plan to have it built. Plan to reuse some fabulous old redgum stairs and chop off the rotted ends. The first 3 steps are winders and I am going to have these in rusted iron with the centre core cut out and a redgum slab dropped in. I thought I would have too much rusty iron so planned to have cut outs of leaves around that, but have found a picture of such a staircase and I dont think there will be much room left. Anyway, has anyone any tips as to what to watch out for, my son in law built one himself and had to redo it as the measurements were wrong and the stairs didnt line up at the top floor