Graham - the next time your underneath the car here's a shot of the rear axle numbers you are looking for. They are located on a flat section on the bottom of the rear end case, between the bracket that holds the rear spring and the bracket that mounts the rear strut rods. Probably full of rust, grease or both.
The codes for my '73 are as follows, your '70 should be similar.
AA = 3:55 gear ratio
W = Warren Mi. Plant
340 = 340th day of production (12/15/72)
E = Eaton positraction
1 = 1st shift
There are also casting dates on the back cover and main case of the rear end. If original, those dates should line up with the rear axle build date and the car's body build date. I can post pics of those casting dates and their locations if you wish. Good luck !!
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John Sigmund
valkman57@sbcglobal.net
NCRS Member 61302
NW NCRS Chapter Member
Duct tape is the new Black !!