Your terminology is a little confusing. The term "crash box" refers to a non-synchronized 4 speed used prior to around 1947 or so when GM started using the SM-420 synchronized 4 speed. Those two transmissions have completely different driveline-mounted parking brakes, and the 420 has at least two different designs, based on the load capacity of the truck it was used with. Can you post a wider perspective photo of the transmission you're using so we can tell which one you have? The one in Snake's photo is an actual "crash box" and the lever he shows operates a parking brake on the rear axle, not the transmission.
Jerry

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