Call Greenwood Radiator in Seattle and talk to Ron. If you go with him, Ill run by and bring it to Tacoma within a week of recieving it on one of my visits to my buddy Pig.
This is the way to go. Hands down.
Until you see a new heater core next to an original, or fitted into a heater box that actual fits, assume it doesn't.
I haven't seen proof of anything currently being made that fits correctly.
In fact just the opposite.
Here's photos of one for a 67 in a 67 https://www.forabodiesonly.com/mopa...eater-box-replace-core.493297/post-1973902555