It is COMMON for vehicles such as yours with a big camshaft to have issues with not enough vacuum for the brake booster. You need a separate electric vacuum pump.
Summit Racing has them.
Hella 009428087 - Hella Street Vacuum Pumps - Overview - SummitRacing.com
Or ... if you are salvage yard prone ( who is not ?? )
Find a diesel powered GM vehicle with the small 12V electric vacuum pump.
82- 85 Skylark-X
I get them for less than 25 dollars used ... and they work perfect for a over cammed SBC.
I got this photo off the internet
DEUCE .. Moderator