My gown is a sacque back, and it has an open front with robings and a stomacher. It can be challenging to copy a painting, since you can seldom see all the details from the front and back. Mrs. Goldthwait, in Copley's painting, is a rich source of information. She is wearing a brown satin gown with sleeve ruffles, ruching and a decorated stomacher is barely seen, but she is turned sideways enough to tell that her gown has the flowing pleats of a sacque back. Mercy Otis Warren, in another of Copley's paintings, wears a blue satin gown, with much ruching and decorations on the stomacher. It is a bit earlier, and the gown may be a studio piece, as there are other paintings that seem to use the same gown. This gown has many of the same details as the gowns in the Kyoto, Revolutions of Fashion, exhibit book.
I used a gown from Kyoto, p. 42 for the meandering pattern of ruching on the stomacher. I used the back views of the 3 sacques pictured on p. 52 to work out how to do the pleats and sleeve tops. Good close ups really help, when you haven't got extant artifacts at home to examine! I also used the schematic from Baumgarten's Costume Close Up for the construction, as well as Janet Arnold's Patterns' of Fashion for the sleeve ruffle shape (thanks, Sue Felshin, for lending me the pattern you made from the diagram in the book!).