68 Dart Heater Core Replacement

I'm looking at replacing the heater core in my 68 Dart GT (the car is a 273 with A/C)
I need to find out who supplies the best replacement A Body Heater Cores, as I have heard that a lot of the replacement cores dont fit correctly.

As I am in New Zealand, I cant purchase a replacement core locally, and it is extremely expensive to send the parts back to the US, if they dont fit correctly.

Has any one had good or bad experiences with replacing the Heater Core in a 68 Dart, (or similar A Body)?