Is it absolute necessary for that to remain as copper?
You could use a service weight gasketed hub to make that connection...but I can see the application now.
Cut you up some old rags in thin strips, pack that hub to where there's a 1" of space to pour your lead.
Sand that copper and flux it, nothing gets fluxed on the cast iron hub.
That way you can fill that 1" of space with lead which will lock both that copper pipe in by bonding and the grooved upper edge of the cast iron hub.
You'll be melting your whole fishing tackle box of lead weights in that fashion with the space you have to fill.