What Mr. Karg says, and if hood clearance allows it, try both a one and a two inch open hole spacer back to back at the track and see what happens. But on a 360 pulling to 6200 I would be inclined to run less plenum volume and port match either Weiand Xcellerator (Rated 1500-70000) or Edelbrock Torker II 340 (Rated 2500-6500) with a set of Felpro 1213S3 intake gaskets to block the exhaust crossover. One member I know of picked up .3 over an air gap with an original Torker port matched in a similar combination in a 340 Duster.
