You can also check out Reconstruction Era Fashions, which covers late 1867 and 1868. While there probably aren't many suitable patterns in there, I seem to recall seeing some discussion of wash dresses in the text. It's all pulled from original issues of Harper's Bazar, which can be found without the pattern supplements here.

You'll probably be safe going with a gathered round waist, raised a bit above the natural waist and a gored, pleated skirt worn over an elliptical hoop and coat sleeves. Practically everything I've seen from this period has coat sleeves, the variation seems to be in the degree of fullness at the elbow.