Provided that your carb id set up correctly you might want to check your fuel pump. Make sure you have the correct pump...for a 455..not a 400 or 350. I had a 455 in my 67...the 455 used more than the stock 400 fuel pump could produce. I ended up with an electric pump...problem solved.
Most likely you had a problem that whatever you did when you installed the electric pump fixed it. A stock v-8 mechanical pump will provide the fuel you need for a 455 with one carburetor up to, at least, 850 cfm.
