I am running a 261 bored .060 over with stock cam and a holley/weber 32/36 conversion. I have adjust the jets up and down and this is what I ended up with: Idle Jets - 80/75; Main Jets - 140/160; Air Jets - 165/180. with the timing advanced about 5 degrees it runs out pretty good and is getting in the neighborhood of 13-14 mpg at 65-70 mph. Here is a link to a pic of my setup, hope this helps.
http://www.chevygmcofkc.org/Gallery/showgallery.php?cat=500&stype=1&thumb=1&si=261_eng_5 57 Mike
