Search Ebay for "SM-318 parts". There are dozens of them, plus several complete transmissions listed virtually every day. I'm always suspicious of somebody who insists the only way to solve my problem is to sell me something expensive!
Jerry

Edit: Just checked- - - -there are 5 or 6 complete transmissions, a couple of low-reverse sliding gears, a cluster gear, a reverse idler, and several small parts kits listed under "55 Chevy transmission". 55-64 transmissions will fit, and the gears and other internal parts on the two different output shaft length units will interchange.
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!