Heater core shot

So the fish is sick.Heater core is done,anti freeze leaking in below the glove box.#-o Question is, does anyone recommend the best one to buy and where. I am in Canada, BC to be exact. My wife works at Lordco so I will purchase from them. Any suggestions as which one I should purchase. I like to replace parts with the best I can seeing I get a good price.
68 340 -S. Any of you Canucks or anyone else have suggestions? Part Number? Are some better than others or pretty much all the same? Will order from the USA if there is something better out there.
Thanks
Spark.