I did this years ago. Long story short, I bought some aftermarket 2 piston calipers (Power Stop?) and they stopped worse than the stock single pistons. I then changed over to manual brakes with a master cylinder and rod from Master Power Brakes (I think), and used the different mounting point on the brake pedal. Still didn't stop well, so I went back to stock all around.
BUT, I never tried the manual setup with stock single piston and standard type pads. It may work better than my manual setup with the dual piston and performance pads.
I also tried a vacuum pump and hated it - too noisy and results weren't that great.
I wish I had a better answer for ya. Try the Chevelles.com, betcha somebody will chime in.