MAGS--they do make C5 rear conversion brackets, but the kit does not retain the park brake. I don't know if the KORE3 kit will fit the B-body. I suggest you contact them, or check it out on their website.
As far as rotor machining, I'd say in the $20-$30 range, maybe as little as $35 a pair, but it will vary by location, shop rate, etc. I could be way off...
If you're interested in what else is available for the rear brakes (what I'm using), contact me by PM or e-mail.