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On my 1977 nova I was trying to find the casting number but the only number I found eell could find was 12345 and thats it
That's not a useable number. The casting numbers will tell you the basics, but the codes usually found stamped into the axle tube will be more specific as to what the rear end was originally in.

Start by finding out the rear end type. In your case it'll probably be the 8.5" corporate 10-bolt. From there, find the codes stamped into the axle and take a look here for links to what they mean.

Rearends got changed a lot through the years, so the ratio from the stamped code might not be what's in it now. To find the ratio now in it you can take a look here.
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