Carb. Mechanical or vacuum secondaries?

Trying to decide on what carb to use on my 5.9 Magnum build. I'm thinking 650cfm as it's not a strip car, but more a performance street. My question is, would you use a mechanical secondary set up, or vacuum? Car is 67 Dart. 3:23 suregrip, havent decided yet on the torque converter but thinking it will be 2500-2800 rpm.

Thanks!