The
Engineering Features brochure from Chevrolet indicates that stiffer rocker arms and shafts were used with 261. See pages 59 and 63 of the linked document. It does not address the shaft offset. I was rooting through my parts collection and could not find any 235 rocker arms to compare to the 261 arms.
Tim Lederman, of the Stovebolt Page, has assembled a most valuable
website.