Thanks Wade for the additional info. What numbers would I be looking for that would be correct for 440 6-pack with a four speed? Year of car would be 1970 with like an Min October build.
The 440-6 4-speed Ebodies required the Dana60 rear, as part of either the A33 Track Pak or A34 Super Track Pak axle packages. If the car had the A34 version then it came with the axle pictured above, the 4.10 with 10 drum backing plates and 10" drum brakes (because A34 included front disc brakes as standard). However, if the car was coded for the A33 axle package then the Dana came with 3.54s and one of two brake sizes. If the car was coded for A33 and manual front drum brakes then the rear axle got the 11" heavy duty drums. If the car was coded for A33 and the optional power front disc brakes then the rear axle got the 10" drums. There were only these 3 variations for Dana60 axles on E-bodies, 4.10/10", 3.54/11", and 3.54/10". If you know which axle package and front brake style your car came with then we can know which of these 3 Danas you need.
Datecode-wise I can only give my opinion, but I'd say you'd be fine at 1-2 months earlier than your car's scheduled build date.
Sorry, I'm not sure about 8 3/4 datecoding.
- Wade