Putting in a metering block (the wide one) would only give you the ability to re-jet the carb using standard screw-in jets .
"Double - Pumpers" are just that ... they have an accelerator pump on each float bowl, and have mechanical secondaries.
Two accelerator pumps are required in order to provide enough fuel to the engine during the transition phase (the time between when the throttle plates open and the venturies develop enough velocity to draw fuel through the main jets. There is also a power valve that supplies more fuel in between the accelerator pump and the main jets.)
Although "double-pumpers" DO have thier place -- on a manual transmission drag car for example, vacuum secondary carbs are a usually a better choice for automatic transmissions, and street-driven vehicles, and are also more economical.
Holley does make a selection of secondary springs which can be used to tailor the opening time and rate. Most carbs come "out-of-the-box" with a black spring ... which is the heaviest. My personal experience is that the purple spring (which is on the light side of middle) seems to work best on a ~350 cu in with an A/T, ~3.50 gearing in a mid-size car.