I've got a 1959 261 in my truck. It has the air cleaner I can get measurements if you like but as tclederman wrote you'll have to just size it for your carb.
That was the first year for the 261 so it may have to locate that engine for the air cleaner engine. May be they increase carb size but I don't actually know. Or should try years 59+
Might try here for some info
http://chevy.oldcarmanualproject.com/

1947farmer

1947 2-Ton Chevy
Story in the Stovebolt Gallery
More pix in Photobucket
1977 3/4-ton
1985 2.5-ton
2- 1947 Allis WC
1- 1949 Allis allcrop
1- 1955 Allis WD-45
1- 1976 Allis 7040

"If it ain't broke you ain't trying"