I moved this out of classifieds because it is not a for sale or want to buy ad.
The 573 was the best of the 3 x 2 intakes that were available for testing. However, 2 x 4s is better, probably because of more carburetor cfm capacity, followed by 1 x 4 being the best, because of a less complex port layout and more than adequate flow from the plethora of carburetor options. The 573 seems to produce real nice low end torque which possibly makes it one of the best options for street use.
I see the 573 much less often than other intakes on eBay. I don't know that I'd go so far as to call it rare, but it is a little harder to find.
As far as the flow limitations of the old 2 barrels, Strombergs are still produced and they offer a Big 97 variant which is rated at 250cfm. It is not stated at what vacuum. If that is like most 2 barrels, then 3 of them work out to about a 530cfm 4 barrel equivalent and if they are rated like 4 barrels, then it is 750cfm equivalent. Either way is quite a bit more flow than the original 2 barrels and may close that gap between 3 x 2s and 2 x 4s.
Here is a link to 3 x 2 testing
http://www.eatonbalancing.com/2013/12/18/y-block-ford-3x2-intake-testing/This show a comparison of 3 x 2, 2 x 4, and 1 x 4.
http://www.eatonbalancing.com/2010/12/15/ford-y-block-aluminum-head-testing-part-ii/Much more good reaeding is here
http://www.eatonbalancing.com/2000/03/24/quick-index-for-eaton-balancing-articles/
Lawrenceville, GA