I have seen a few early brake systems retro-fitted to self adjusters. I'd suggest finding a similar size later model Bendix brake setup in a salvage yard and either scavenging the parts or at least taking a series of detailed photographs to guide you in the conversion. The cable-type self adjuster system would probably be the simplest to transfer. There's a cable attached to the top anchor pin that is routed over a guide on the rear brake shoe that operates the ratchet pawl of the adjuster, along with a different lower spring assembly and a fine-tooth star wheel. It doesn't necessarily need to be for a GM brake- - - -Bendix made similar systems for a bunch of manufacturers. Anytime the shoe assembly rocks back far enough during a stop in reverse to pick up a tooth on the starwheel, the shoes get adjusted out a few thousandths of an inch. That continues as long as there is sufficient brake lining material left to be effective.
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
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