There are lots of quotations by the directors (Corbucci, particularly) culled by the newspapers of the time.
Oooh, this sounds like it might be worth picking up for the quotations alone.
Fistful, Django, Quien Sabe?, Django Kill, Trinity, Sonny and Jed.
but wtf the scatterbrained Sonny and Jed? As an example how bad the SW had become later?
The main sources are a panel featuring Leone, Corbucci, Vincenzoni and Tessari published on L'Europeo, 3 February 1966. And this book:
I presume you have this one:
The Corbucci's book I posted includes Corbucci's Notes Towards an Autobiography which I think ought to satisfy most of your curiosities. It also includes a lot of pictures.