Those plates are very similar to the ones for the tubular shocks used on the later trucks, except for being flat rather than angled forward to match the shock that's angled toward the front of the truck. The Filling Station has the later ones, which could probably be heated and flattened to work with the older shocks.
LinkOr you could fabricate a pair from flat steel plate. The ones from the Filling Station look like they're cast steel, as were the originals on my '51.
