Your first troubleshooting sounds reasonable, so I'm surprised you did not find a bad brake booster that was screwing up the vacuum.
However, if its not vacuum, then maybe its electrical. Something may be grounding out when you press the pedal, or a sudden electrical load may be causing such a draw on the alternator that it pulls the idle down very quickly and it dies.
- Hold the brake pedal down and start the car. Try this in several positions of the pedal travel. What you are looking for is a point where something grounds out and causes an electrical problem. If you have an aftermarket tach, take a close look at the wiring to the HEI to make sure you don't have a wire running next to the brake pedal arm that is grounding out the ignition.
- Try pulling the fuse for the brake lights to see if it resolves the problem. I don't think the lights could put enough load on the alternator to pull the idle down significantly, but maybe something else (add-on 3rd brake light?) is drawing excessive power. Check for any relays that might be connected to the brake light circuit. If someone has added any aftermarket security devices, that would also be a good place to start.