Would you consider re framing the art?

I would lower the art and put the clock off to the left. Then add to candlesticks to the right side of different heights. Remove bowls.

Maybe even put a smaller shelf above the artwork, and the clock on that. It might look better framing the artwork between the mantle and a shelf,instead of having the clock between the mantle and an artwork-its a little small and looks unbalanced that way

What kind of frame for the art were you thinking?

I was thinking a dark/black? frame would make the art stand out more.

The art just needs a fatter frame and slightly less mat showing . . dark wood to echo the mantel is one option. The overmantle suggestion isn't far off since these proportions are made to be sort of colonial . . but that will be a more formal look and requires painting brick,

Since the brick tones are repeated so beautifully in your art, my advice is - get a dark wood frame stained close to the mantle tone. Not chunky and rustic but not ornate - very shaker ish. Take everything else off the mantle but your clock.

At holiday time, hang a wreath framed in the area between the firebox and the mantel, and if your decor is homespun, have a plain twig or eucalyptus ring that hangs there year round. It will echo the roundness of the clock face.

Reframe the art as suggested above and lower/lean just above fireplace. You need some bulk on either side of art. Do you have other standing clocks...bigger? Maybe some fat and happy pots? The tall lean fireplace and tall lean art need some width.

I love the idea of a shaker frame-it would help balance the smaller painting with the larger brick area. I am not certain the colors of the brick are repeated in the painting-the brick is more orange. An overmantle with white painted brick would make the room look far taller, larger and more spacious, and act as a natural second frame to the artwork with a shaker frame, accentuating the piece. That is, if you are comfortable painting the brick...

Thank you all for the suggestions. I like the idea of re-framing the art, and will do so. I do like the way the brick color is echoed in the lithograph, and am not ready to paint the brick! I will come back with my newly framed art.

The wreath idea is a good one! And so is adding other clocks to the mantle.

Mat and frame the art and rehang over the fireplace with the required clock. Add drapery panels on the windows using a color selected from the framed print.

The new mat is lovely. The clock in the center detracts from the picture. Can you move it off to the side?

The top board on the mantle needs to be more substantial. A lot of brick and a small edge. I would add some crown molding below the edge.

See pictures for variation

I'd love to be able to say I re-matted the art, but it is just photo-shopped in Beverly's comment! Nice job!
The mantle is made of slate or bluestone, so I don't know how I would add molding.
I do appreciate suggestions, and when the art has been reframed I will ask for more.

I think I would find a different place for the clock (sorry).

Oh, I know, but it is a non-negotiable item in a 36 year marriage.

Always choose your battles; some things really are not worth arguing about.