The 1954-up bell housing has a redesigned mount and a different angle cast into the housing (around 45 degrees). That's the one that requires the long bolts and sleeves. It also needs a different crossmember with angled, not flat mating surfaces for the mounts. The later style mount is far less likely to deteriorate and separate than the vertical ones.
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!