Yeah, I guess it does take both carbs. The Holley 4175 you mentioned is a spreadbore replacement for a quadrajet or Thermoquad. The quadrajet-type is a decent carb, assuming it's tuned correctly. The small primaries with the large secondaries and vacuum sensitive air valve is actually a pretty good idea. The secondaries only open up as much as the engine requires. If adjusted correctly, the secondary air valve prevents the engine from bogging when you stab the throttle.

Another carb to consider is the Edelbrock Performer. They're selling the heck out of those. I talked to about a dozen guys at the T-bucket Nationals in July, and every one of them swore by it. Their 600 CFM, electric choke would work.

One of the Holley 4150 series - about 600-650 CFM would work too.