What's needed to get accomplished is get the C band feed throat in the same position as the existing Ku feed and have means of adjusting the fore aft position.
If I were in need of doing this, think I'd put a string from the top lip to the face center of the Ku lnbf (placce a mark or tape) and continue the string to the bottom lip. That will locate the vertical position as well as the focal length. Leave string attached to the dish and remove the Ku lnbf and mounting hardware.. Use the strings lnbf position to position the C band lnbf. Tune up the focal length watching the Q meter then fashion mounting as necessary.