That brake isn't particularly unique- - - -lots of the SM-420 transmissions used them, including the 59 3800 model 1-ton I owned a few years ago. It shouldn't be difficult to find the missing parts.

However- - - -there's another issue you need to be aware of. The wheels on your truck are the Firestone RH-5's, the infamous "widowmakers" that have been implicated in dozens of injuries and deaths from exploding wheels. They're even illegal to run in some places. The problem is that the rims split in the middle, and it only takes a slight amount of rust damage or other deterioration to make them very unreliable and possibly deadly.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!