Not an expert but you may want to check you carb number on this chart to get a starting point. I don't use power valves myself as my junk is strip only and I leave off the brake.
http://tgsperformance.com/holley.htm

1970 Duster
Edelbrock headed 408
5.984@112.52
422 Indy headed small block
5.982@112.56 mph
9.42@138.27

Livin and lovin life one day at a time