Well, I finally dug out some of my brake stuff to find out what fits together. So far i have this setup which looks like it might be promising..
That's a rear 12" C5 rotor with my old stock Alero front calipers. There's a little difference in the spacing in the mounting holes, so an adapter would have to be fabricated to get it to all work perfect, but it's something.
I have some other parts to try too (I have rear F-Body calipers/brackets as well as rear C5 calipers/brackets)