How can I hang large art above board and batten?

You can put a board at the top of where you will hang the artwork that is the same thickness as the trim on the board and batten. Then it will be evenly spaced top to bottom.

I would hang it above the batten, https://www.shanty-2-chic.com/2013/02/diy-wall-art-gallery-frames.html

https://www.finearttips.com/2012/06/diy-build-an-art-gallery-hanging-system-for-around-100/

Add a picture rail and hang the picture using wire or ribbon; http://lovegrowswild.com/2015/07/statement-hanging-art/

https://www.pinterest.com/pin/390124386441736461/

is the batten low enough you can put a picture shelf on top to put your picture on?

https://www.google.com/search?q=images+of+board+and+batten+interior+walls+and+picture+hanging&client=opera&tbm=isch&tbs=rimg:CQ3AhmWy2F8pIjjGsYj7RlUKivEfsRwQskFVWKbsp9SsYPojSqbOREbGZvszRIjRAXz-WOAVrhgtTlMYrsZxyG9qVyoSCcaxiPtGVQqKEeIwoE7UCGcMKhIJ8R-xHBCyQVUR2NZ51KTUvo0qEglYpuyn1Kxg-hElTdt10bBntCoSCSNKps5ERsZmEe89McBZjis-KhIJ-zNEiNEBfP4RAbvymUEOofMqEglY4BWuGC1OUxF05bvTzJ7r-CoSCRiuxnHIb2pXEZDeZlHaO45P&tbo=u&sa=X&ved=2ahUKEwj3laaE_rnjAhWKxlQKHd42ClEQ9C96BAgBEBs&biw=1473&bih=695&dpr=0.9#imgrc=140rQpJcy8ykXM:

Would be so much easier to determine if we could see a pic of the space. As the a collector of art and having an artist husband, we have art hanging from ceiling height to very low. Art can always be rehung. Remember the art itself, not just the size of the art, must "go" with its environment.

I would use a French Cleat. The cleat is made of two width of picture pieces of wood. The other sizes would depend on how heavy the picture is. One side of each piece is cut on an angle. The two cleats will rest against each other here. One is attached to the wall. One is attached to the picture. Make sure both are level. The wall cleat has the point jutting away from the wall, the picture cleat has the point jutting towards the picture. You hang the picture by setting the picture cleat into the wall cleat, gravity does the rest.