Refresh my memory-weren't you the guy in Weymouth that had the '67 Fury vert burn up on him?
Anyway, it's a double core as opposed to a single core. Easiest way to start is to remove the front seat-I'm not kidding! Then you need to start disassembling the hater box to get at the core-either that or just take the whole box out of the car.
The cores are the same as the aftermarket B body ones offered, but the piping configuration is different I think. I would remove the old one and take it to a old time recore shop and have it recored. At least them you'll know it fits.
My friend and I replaced the heater core on my '68 Newport Custom with Autotemp-it's no fun!
Good luck!