This Derale fan features a symmetrical housing so that it can be mounted on either side of the radiator for use as a pusher fan or puller fan. They mounted it on the rear of the radiator as a puller, which is more efficient. Pusher fans have the disadvantage of physically obstructing airflow to the radiator, and pushed air is more likely to bounce against the radiator causing turbulence.
Getting the most advantage of a puller fan requires a shroud between the fan and the radiator. Since the whole purpose of the fan is to draw as much air as possible through as much of the core as possible, they chose the largest diameter that would fit. But without a shroud, even a 17-inch diameter round fan would miss a lot of potential air from the four corners of the 203/4x193/4-inch radiator, so they had U.S. Radiator create an aluminum shroud before shipping the fan and radiator to Hot Rods by Dean in Phoenix, where they were waiting to install it in the Chevy.