Some of us install the later Chrysler AM/FM/tape/CD unit, which fits in just right, and hooks up the same, except for a fiddle with the lighting, no big deal at all. You do have to use the radio with the square cornered face plate, maybe with minor trimming but no big deal.
Otherwise, you can use pretty much whatever you want, the car radio guys are pretty good at mounting almost anything. It is just a Chrysler hook up, two big connectors.
I think the rears are 6x9, and the fronts are 5-1/4? Just do a google search to be sure.
1989 Milano, Shankle Sport
1991 164S, stock
1994 164LS (~Q)
1972 Morgan 27
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6