I had a similar failure, but the neck broke about 3/4 inch from the end of the pipe. I refitted the hose as there was enough pipe left for a good fit - 3 summer weeks now and it still holds.

i digress

using copper inside the neck of the radiator will be problematic in the long run as copper expands rather nicely and may cause extra stress on the pipe. AFAIK a thin aluminium 'pipe' is used in the new radiator to strengthen the neck ....

Also, remember that the coolant might corrode the copper fairly quickly, or that introducing copper into the cooling systems might react with the steel (block) and aluminium (head, pump and radiator) the coolant comes into contact with.

maybe some chemical expert might come up with accurate answers ...