Classic/Antique Car Repair: Brakes, power brake booster, 007 james bond

Question
QUESTION: I purchased a 1981 Chevy El Camino. The previous owner swapped in a hi-preformance 350 engine. The motor has a 30 overbore, high-lift cam (duration unknown), high rise intake manifold, 750cfm carburator and 2 inch headers. The timing is set with a 20 degree advance just to enable starting and hold an idle, anything under 20 degrees and I can't start the car. My proble lies with the braking system. I'm told that I'm not getting enough vacuum to the power brake booster resulting in a soft pedal and poor braking. I've replaced the brake pads,lines and bled the system. This gave me stopping power for a couple of days and reverted back to it's original problem. Any ideas as to what to do next? Is it possible that the booster is not getting enough vacuum? Thanks in advance, Al

ANSWER: Soft pedal and poor braking as a symptom do not indicate booster problems. Can you pump the brake pedal up when this condition occurs? Let me know.
Brad

---------- FOLLOW-UP ----------

QUESTION: Thanks for the responce, Sorry to say that there is no change by pumping the pedal. It stays soft and the brakes do engage with the pedal practically on the floor(about a half inch). The master cylinder is full and there are no visible leaks throughout the system. Thanks again,Al

Answer
If I were doing the job I would start by pulling all 4 wheels and checking the installation of all components. Then make sure the rear brakes (drums) are properly adjusted and bled. Don't forget to make sure that the parking brake cables are free and properly adjusted. On the fronts make sure that the pads are seated properly in the calipers and that the calipers are not worn and wobbling on their mounts. This is a 007 James Bond job.
Let me know what you find.
Brad