Take a look at the 0-4776C carburator at that Holley link below.
http://www.holley.com/Catalogs.asp
It's a 600 CFM double pumper with mechanical secondaries that is identified as Street/Strip.
I have dual 660's with mechanical secondaries on my car, and I love them. I have always preferred mechanical secondaries as my right foot controls when they open.
Yes, they do suck down more gas, but I didn't buy this car for gas mileage.
Just my opinion.