My first thought would be Carafa III (dehusked/debittered) and scale the amount based on the difference in lovibond.
EDIT: On second thought, I'm not sure what you mean by dark wheat. I was thinking something like chocolate wheat (a roasted wheat malt), for which Carafa (though now that I think about it maybe a lower Carafa like Carafa I) would be a good substitute, but when I searched "dark wheat" in Google all I came up with was Weyermann dark wheat, which is around 10L. I'm not sure why you would use that in small quantities. Could you post the full recipe?