From the online Holley site
"QUESTION How do I know if a vacuum or mechanical carburetor is best for me?
ANSWER For streetcars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a double pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes. "
So gang this is how I picked out my carb I think a cobra fits description #2
The key is Vacuum!
Here's a good tuning page for Holley.
http://corvetteactioncenter.com/kb/q....php?qstId=434
Tim