Definitely 12-bolt all the way. Ratios were 3.31:1 standard, with 3.55, 3.73, 4.10 optional; if you had a close ratio you could also get 4.56 & 4.88. They also used the rear frame reinforcement that the SS396 used.
The 327/275 was 10-bolt, unless you ordered the M20 4-speed (Saginaw 2.54 1st). Axle ratios for the 327/275 ranged from 3.07-3.73.
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