Hi JJ,
In 1984 (©1983), the
Chieftain Brochure shows that the AM/FM Cassette was an option. For some reason, the 1985s don't show-up in the brochure links.
My guess is that you can buy a speaker which will fit the cut-outs, both front and rear, which will out-perform the forty-year old speakers which are in there. As far as the radio is concerned, I believe you will need the single-din size. Please let us know what you buy and install. You might just go to a car stereo place to buy it from there.
Welcome to the forum.
Eagle5