That part is for a 1.5 or 2 ton truck
Your Canopy Express is either a 1/2 ton or a 1 ton.

Let us know the year/size in each new thread/topic.

I would inspect the brake cylinders before buying parts - it might be pitted beyond repair/rehab.
In fact, the complete wheel cylinders might be readily available - I would replace all units (to be safe).

The on-line "reference" for 1953 truck brakes is unreadable.
Here are specs for 1952 truck brakes and 1954 truck brake specs.

Also, measure what you have - someone might have cob-job changed it over the years.

Congratulations to your son (and you) obtaining a Canopy Express - a very rare model.