I installed a Restomod Bantam II unit in 2511. As David mentioned, this is far from a trivial project.
that unit has provisions for three zones, dash, floor and defrost. in my installation I removed all the outlets for those three zones and created my own outlets and distribution plenums.
I also modified dash structure to allow the installation of outboard vents.
rebuilding the OEM heater box involves no engineering or fabrication, just component replacements.
pick your battles. 😉
Larry