You've still got your Ku single-polarity LNB and feed on the dish (in the pictures).
So, take a string and mark/hook it to the bottom of the dish in the middle.
Run the string to the face of your feed and mark it with a magic marker, put on a tie-wrap, or make a knot.
Continue the string up to the top of the dish and mark/attach it.
Install the C-band feed to the same neighborhood as where the knot is located on the string.
This will give you distance from the dish and angle to the face of the dish.
Of course, you may need to move the feed toward/away from the dish as much as an inch, but this'll get you started.
This following idea will complicate tune-up, but it might still be useful after you determine LNBF and scalar placement.
Caddata came up with some adjustable brackets to float a scalar at the end of three feed struts.
Here PopcornNmore has some
beautiful pictues of the design, and discusses adapting it to conical scalars, too.
I see your Prodelin has
three small support arms, so I was reminded of this design.
I've even
tried it myself, just based off some of Popcorn's pix (I couldn't find Caddatas drawings at the time)
More notes:
Quote of ACradio and MikeKohl on scalar tuning:
1.2 Meter miniBUD Modified Scalar Ring