66-67 273 four barrel intake.....

....what's the best fitting carb....my E-brock 600 manual choke doesn't want to open up.....the bores in the intake aren't big enough? I've noticed that some people aren't running an adapter....so what I'm I missing. Does it need the 500 cfm carb to work are the throttle plates the right size? Thanks in advance for the help.