A substantial portion of the Howells family's papers is held by the
Houghton Library at Harvard. The main segment is described at
http://nrs.harvard.edu/urn-3:FHCL.Hough:hou01480 (the MS Am 1784 series),
but there are several other segments. I'd recommend searching Harvard's
OASIS catalog, to one slice of which the URL above points.

Best,
Sharon Goetz