[SuperHart]

As you probably expected, there are many different opinions about dual quads. First, you need to calculate the air volume requirement of your engine. A 460 cid engine at 6000 RPM uses approximately 800 cfm of air assuming 100% efficiency (which most engines are not). There are several calculators online, one being on the Holley web site. Once you calculate the air requirement you can choose carbs. Using vacuum secondary Holley carbs is always a good choice for the street since the secondaries will open on an "as needed" basis. This characteristic should lessen the risk of "bogging" with banzai pedal mashing. In addition, vacuum secondary carbs allow you a little more wiggle room when choosing carb cfm size. You could probably do very well with a pair of Holley #8007 390 cfm or #1848 465 cfm carbs. The #1848s from Price Motorsports ($775 for the pair) come with the Ford style throttle arms and proper secondary vacuum covers for dual quad usage. Either the 427FE low riser or medium riser linkage should work well. The #8007 carbs are usually available used for less money and work well when set up properly (1-2 steps richer primary jetting and vacuum pot covers for dual quad use). Top off your setup with a pair of Stellings and Hellings air cleaners for the original Cobra look.

As correctly stated by jwd you can actually get better gas mileage with two smaller carbs using progressive linkage on the street because most of the time you will be running on the primary carb only. You will love the look of dual quads sitting atop your engine and they really aren't difficult to keep in reasonable tune.