You can get a reprint of a performance book for the 235/261 series inline 6s by California Bill Fisher. It was written in the 1950s, and is out of print, but still readily available today. It's called "California Bill's Chevrolet, GMC & Buick Speed Manual"
I have a copy and in my opinion it's a great book - simply written with no unnecessary complex jargon for us simple types.
-magic mike-
