Ya don't sell the stuff.
Almost every time I sell something truck wise I regret it. Especially anything related to the NLA or soon to be NLA parts. And as I am sure you know, some of this stuff that is still available is NLA to the masses unless you just how and where to get it. Granted it...
I have a 4L60e with a shift kit and tune, in front of the split case. Best auto I have ever had. It doesn't "coast" like so many other Chevy autos I have driven.
Cheers