1968 Buick Riviera Heat System Restoration

The previous owner told me, that the heater core was leaking, and that the core therefore was disconnected from the water system.
As the car arrived with me, i found what you see on the picture. Thats it....
The owner also said, that the car was not equipped with factory installed AC, and still is not of course having AC.
Having no heating, is not an option...
Of course this brings up many questions.

First of all, i cant find this heater core type in the online stores i know. All i see on the pictures got much more narrow distance between the tubes, even if declared for not AC equipped cars.

Second are the tubes connected and some valves in between, which may be missing as well. I have the Service manuals already here, and can figure out the "schematic" of which connects where. BUt unfortunately i have no online part guide to obtain these parts. Getting hold of scrapped car to donate these components here in Germany, is nearly impossible

Last but not least the service manual says, that you have to remove the right fender to replace the heater core. Is that really necessary? I checked for some instructions online, but only came to found for cars with AC, which my car is not equipped with. All these instructions included the poor guys needing psychotropics, after the work was done, as they took out the heater core from the inside of the car. Others had recommended to cut out a piece of the housing to get access from the engine bay. What is the best method?

More questions to come, for sure! :)
Thanks for your help!

Best regards,