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your post does not say if you wish to stay with the stock planetary gearing or convert to derailleur gearing
for derailleur gearing you would need to decide if you wanted to go with single plateau or multi plateau drive
the most common nutted spindle ("Type II") for the single plateau conversion you describe would be the 5N
for a two plateau arrangement the most common woud be the 5S
readers would be able to assist you a bit better if you could give some additional information
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