[blucar]

The vacuum ports on most EFV8 intakes are generally 3/8" to accommodate vacuum for the distributor advance and wipers. The vacuum to operate the Columbia OD is tee'd into this port which is located below the rear carb on my Offie Super manifold.
Generally the engines for the larger trucks have a larger vacuum port to accommodate the vacuum assisted brake booster and/or 2spd rear axle.