Data from the book, Chevrolet By The Numbers, by Alan L. Colvin:
the following is the same for 10-bolt and 12-bolt Camaro axles from 1967 to 1969.
Axle tube flange to axle tube flange-- 54 1/4 inches.
Between centerlines of spring seats -- 42 7/16 inches.
the following is different for 10 - bolt (but the same for 1967 to 1969) axles and 12 - bolt (but the same for 1967 to 1969) axles;
10- Bolt axle center section casting No. 3894859NF
12- Bolt axle center section casting No. 3894860NF