Main case & extension housing #s '70 Nova M21

[rszmjt]

Originally Posted by Too Many Projects

The 661 trans all came with the 26 spline input and 32 spline output to reduce needed parts and inventory for GM. I would think many 660 trans were replaced with the 661 later on to gain the strength of the fine spline and larger output yoke. There certainly may have been late assembly line 661's installed as the stock of 660's ran out, to keep the line moving. Not unusual for that to happen in a changeover year. How many and when would be for better informed people than me.
My Feb built '70 LS5 Chevelle came with a 660 case, but as an M22, it had the 26/32 parts in it.

Thanks, Maybe in Chevelle land but 1970 Z28 used a "661" case with the M22 and it was 10 spline input and 27 spline output, the 71up M22 in Z28 used the "661" case and 26 spline input and 32 spline output with a "764" tailhousing, pretty sure Nova in 70 did too.
FWIW I have never seen a "661" case that didnt have the drain plug location drilled and tapped.