Roadrunner,
I considered this about 8 years ago and decided that an old cassette player would be too frustrating eating tapes.
My 70 came with a mono Becker Europa . I quickly found a Becker Europa II stereo with a separate 7w per channel amplifier which i mounted behind the glovebox. I then bought 4" becker speakers that fit in the foot panels. There are holes designed for those 4" speakers already prepared. The Europa II had a DB7 port on the back that was used for an external tape unit, either 8 track or cassette. That DB7 port can also be fitted with a 3.5 mm female plug (copleye kiy was available on ebay)that fits the output from all cellphones or ipods. We installed the 3.5 plug end under the dash light and used a male/male regular audio cord to plug into the phone. Through your phone, you can play any audio source you want, without the frustration of the old cassette tape player, is fully authentic, and extremely reliable.