How much weight can you shave off of an A-Body...and still have it look/drive like an A-Body?

Start out with a very rusty car.
Then reinforce the areas of concern with old bed frames, exhaust tubing, and cardboard taped to the floors to seal them up.
Whatever is cheap and easy.
Only go as far as you have to go to keep the car from folding in half.
This should shave off ~ 1000 lbs. or so from a factory car.
Then remove all of the factory emblems off it and put Chevy ones on.
People are going to call your car a Nova or Chevelle anyway...