[Double Venom]

John,
You description feeds the physics of the master cylinder with wrong bores, OR YOUR PEDAL IS assembled in the wrong hole creating the wrong pedal ratio.

6 caliber is absolutely 'ridiculous' for these cars. If you're traveling 100 mph + to 20 mph for a hair pin turn, maybe.......

Corvette MC's 7" work well, simple two nut installation. (+/- '87 4 wheel disc). Adjust your rod length to allow less than a 1/4" movement before the piston in the MC starts to move.

Buying a full brake system is always better as a supplied kit, unless you know everything there is about a full braking system.