I've done a GM radio swap and here is my experience with it.
The wiring harness was changed to a 21 pin ( I believe) harness in 1988, so 88 and up are all the same. The 87 and down are all the same. Metra does make a harness that you can just plug your factory buick wiring into and it will fit into the receptors for the 88-up radios. The newer Delco radios are a little thicker than the 2 knob on the left side delco cassette decks. Pontiac radios have red backlighting, so you might want to avoid them because of that, and the fact that their switch gear will look nothing like anything in your cars.
I would try to find an early to mid 90's CD player from a Lumina, Camaro, or Cavalier, and put your original knobs from your cassette decks on it.
Just my $.02
Chris