I recently inquired A tech over at summit and they wrote back saying " Just FYI, the 1965 327ci 350hp engine originally came with a 585cfm vacuum secondary carburetor, and it was a real killer engine.
Either one of the following would do a great job.
For a 650cfm, take a look at
http://www.summitracing.com/parts/HLY-0-80783C/ .. Note this carburetor has a traditional finish, electric choke, dual inlets, and adjustable floats
For a 600cfm, take a look at
http://www.summitracing.com/parts/HLY-0-80457S/ . Note this carburetor has a shiny polished finish, electric choke, and has non-adjustable floats.
Hope this helps.
If I go to big in cfm , wont that cause a problem in running too rich and hard on gas ?