In many cases high performance engines will not have enough vacuum for the brake booster to operate correctly. The system you installed should be more than adequate and require only moderate pedal pressure to stop at high speeds. You have a vacuum problem and a vacuum booster and possibly a storage canister may help. Check out www.abspowerbrake.com. They offer an electric booster that uses no vacuum. It is essentially the system used in late Corvette's. It's pricy but would eliminate any vacuum issues.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine