Cowdoc,
The lowest ratio you can go to by just changing the ring and pinion is 3.23. If you change the differential case ( the part that the ring gear bolts to, NOT the axle housing) then you can install any ratio from 2.56 to 4.33.
The following information was gleaned from the Pontiac Master Parts Catalog(1972)
There were three differential cases used on 1966 to 70 "A"(Tempest,LeMans,and GTO), "F"(Firebird), and "G" (Grand Prix) bodies.
# 9782417 was used with 2.56(41-16)and 2.78(39-14) ratios.
#9782418 was used with 2.93(41-14),3.08(40-13),and 3.23(42-13) ratios.
#9782419 was used with 3.36 (37-11),3.55(39-11),3.90(39-10),4.11(37-9), and 4.33(39-9) ratios.
Posi axles also followed the same pattern with three different diff. cases.
From what I can see, any complete set of internals from any of the above mentioned cars should interchange. If anyone else feels I'm wrong please feel free to correct me.
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