Fuel pressure is critical when you change from a stock carburetor to either a Weber or Holley carburetor. The Weber carburetors are very sensitive to fuel pressure. The needle and seat will not take more than 2.5 - 3.5lbs of fuel pressure. If higher fuel pressure is attempted, the carburetor will flood or poor running condition and poor fuel mileage.
Holley 350 & 500 cfm carburetors will work with a slightly higher fuel pressure. They will take 4 - 6lbs of fuel pressure before the needle and seat will not hold the pressure. They will also flood if excess pressure is applied. The best way to control the pressure is with a high volume low-pressure regulator and a pressure gauge, see Part# 1035043. Need to tell how much pressure you are running, see Part# 1035056 for the Fuel Pressure Gauge kit with regulator.