Technical Master Cylinder size and Braking

Not knowing the tune up, we installed a Tom Langdon’s ignition on our street GMC 6. He said to use manifold vacuum. We could never got the engine to idle under 900 and had to go into 1st for your typical right street turn. I decided to change the vacuum to the ignition to Venturi.... totally different car. Idled at 600, no down shifts to 1st.... Just something else to think about..