ISO
I don't have a lot on the 336 but there were some pretty good reviews in Flying magazine back when Cessna introduced the first Skymaster. These past issues of Flying are available on Google Books
http://books.google.com/books?id=8BV..._issues_anchor
This is the very first review that Flying did and it does give some good detail of performance.
http://books.google.com/books?id=ebV...page&q&f=false
The 336 is quite a different airplane than the 337. Cessna made many changes to the 337 based on that first few years of testing the 336. I have a history paper that was posted for the 50th Anniversary of the Skymaster last year that I will post here again. Gives all the ups and downs of the development at Cessna.