The first factory multiplex/stereo radios were in the 1968 model year. 8 tracks were available with the normal AM radio or were with the AM/FM Stereo radio, as an indash unit. From looking in my copy of The Standard Catalog of Chrysler, it looks like 1968 was the first year for the 8 track tape units as factory equipment. The aftermarket units had been around since about 1965 or so, first as 4 track units that were quickly replaced by the Lear 8 track units.
The sound quality of those 8 tracks can be surprising, compared to the later model cassette units. They could be troublesome, though, if the drive belt ever started having problems or the tape cartridge itself had wear issues. The tape was specially lubricated as it had to slide over itself inside the cartridge.