Here's a bolt-on style dual-master/balance bar setup from CNC
The 241 series uses Mustang style bolt pattern, which is 3 1/4" C-C. The 242 is a 3-bolt pattern, and 243 is a 4-bolt pattern. I think it would be pretty easy to adapt the 241 to a GM car, but the $391 price tag is out of my league.
http://cncbrakes.com/cncbrakes.com-a...240&subseries=