How much does the van weigh? Auto or manual transmission? Gear ratio? Have you driven a vehicle with a double pumper? The velocity of the air drops significantly when the carb is cracked wide open (2 to 4 bbl), which doesn't hurt a light car too much but on something heavier it will bog. The vacuum secondaries will only open if the vacuum on the carb is high enough to demand it so the velocity of the air is less affected with a vacuum secondaries carb. Read up about it on the holley website, A double pumper isn't always gonna make more power than a vacuum secondaries carb.
BTW what are you using for metering rods and jets right now?
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