I ran into that problem when I needed a rear main bearing set for a Chevy big block with the thicker thrust flanges, like you used to always be able to get. The part number is still active, but unless they get at least 100 orders, they won't send it in to have a batch made.
Might be something like that.
This is why I finally broke down and replaced my original rear end with a Ford Dana 60. Other than the lack of a posi, it is nice to have a 3.50 gear set back there! I actually have highway legs now.
I think if you have a 3600, you're almost forced to restofy instead of restore.
