Another vote to send the original core to Glen Ray for repair. They will do it right the first time and even though it will cost some $$$, at least you'll only be doing it once with these guys. Can't say anything bad about them other than they stand behind their work -- and the original tanks that were used back in the day are the best there are and the heater matrix is very repairable with the right guys doing the work. You cannot go wrong with Glen Ray, and no, I'm not the owner -- just a guy whose had them do three different cars radiators and heater cores for me. Turned out perfect every time.......

Good luck on that gorgeous car!!!!!!!!!