wow, I have never heard the trick about wrench sizes to tell the difference between the 8.2 and 8.5..... this site rules! We should have some sort of reference to all these little tricks someday.. they are a lifesaver.
If you know for a fact the rear end is 70 vintage then it is an 8.2. 71 is the intro- ductory year for the 8.5. Try looking at Gto Gs 442. They had plenty of chevy 12's loaded under them. Try hooking up with some members from the south or mid-west.Local salvage yards in those areas tend to have loads of older parts than other regions of the USA. Another alternative is the old 9 inch.
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