A few years ago, I asked my radiator repairer about leaky heater cores, & he said it was hard to work on & quite often still leaked, & the only way he would do it was to buy another new core similar size & adapt that to work. He said to buy whatever was available, not sure if he was just being lazy. I would always worry about the quality of repo ones, but may not have much choice. Up until recently there was still original a/con heater cores available through natra, not sure about now.
Foam goes between the footwell box & firewall, I presume just make it to suit. I recently bought, some for the top of that box, where it screws up to the plenum, off eBay, but could've done a better job myself!

Now, another question on this subject, I've got my footwell box out, & the rubber seal on the flap inside this, the rubber is as hard as a rock, anyone seen replacement rubber for the flipper flap?