Hi there, Lee,
Many of the old lathes used a mild steel mandrel running directly in cast iron. Given proper lubrication, they ran for decades (some are still running). The cast iron contains graphite which has lubricating qualities but oil is still necessary.
If your clamping relies on flexing the cast iron one side of the 'bearing', then the fit needs to be very close – if you try to flex the cast iron too much, it'll crack. That's the failure mode of many of the older lathe head-stocks.
I suggest that one ingredient for your scheme to be successful is that you take precautions to exclude dirt and grit from getting into the clearance (small though it may be) between your cast iron and mild steel parts.
Best regards,
Swarf, Mostly!