[Kenth]

1966-72 Pontiac A-body uses the same rearend width.
1964-65 was 1" narrower overall width.
Only 1964-66 uses the bolted down springs.

For '66, the 10 bolt center casting was p/n 9783393.

For '67, the 10 bolt center casting was p/n 9783393 (no change). This casting was also used for the '67 Firebird rear end assembly, even though the axle tubes were different and the upper ears were not needed/used.

Chevy A-body was different, 1964-67 was the narrower style, 1968-72 was 1" wider width.

Also, NO need to replace the springs using a 1967 and later A-body rearend on 1966 and earlier.

1964-66 A-bodies uses rear springs with one tangential end and one pig tail end.
1967-72 A-bodies uses rear springs with pig tail on both ends and the upper end bolted to the frame.

FWIW