Got Shell 93 Octane and now barely idles 1973 Dart 340

Without that plate the gasket will not seal around the secondary section of the intake. The manifold has both bolt patterns if it is aftermarket. If it is the factory cast and your carb doesn't have both bolt patterns then you need the thicker adapter plate that uses additional bolts that come up for the square pattern. Without seeing the manifold top we can't be sure if the thin plate will work but if it has not been modified from stock it is likely the thin one wont let the primaries open.