If we're talking for street use, my choice for the small block Ford's with an automatic would go to a vacuum secondary. A 347 will run strong on a 600 CFM carb--Edelbrock or Holley--when it's set up and jetted correctly. If I'm taking my small block drag racing, it's dependent on what other components are in the engine, cam, heads, trans, stall speed, etc. it would be either a 600 vacuum secondary on a mild engine, and probably a 650 mechanical secondary on a high output engine... Generally, for me the vacuum secondary has always been the better all around carb with an automatic transmission.....Which is just another reason I usually run 5 speeds with a mechanical secondary carb!!!!!