I don't think, from everything I've tasted, there is a malt that is a sub for molasses. There's an undeniable flavor to molasses that is not found in other sugars - or malts for that matter. That said, I also think molasses is very easy to overdo in beer and can be polarizing, so perhaps a super dark crystal is a better solution anyway. You could try Patagonia Crystal 190L but it'll bring some toasty notes along with the dark sugar notes.