Maybe I'm missing something here, but have you compared the new cylinder with the one on the other side? It sounds like somebody's giving you the wrong part, even if the diameter is correct. I've never seen any American vehicle from the 50's, car or truck, that uses anything other than a regular double-flared line in the brake system. Some front wheel cylinders used an oddball seat for the flexible line, but all the solid-line hookups I've seen on rear brakes use an inverted flare.
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!