I think a brown worsted would be a better choice for a sack coat than a frock. All but one of the surviving original wool frocks I have seen have been made of black woolen broadcloth. Fashion plates suggest that other colors existed but probably in similar materials and dark colors. Sack coats, OTOH are depicted in a range of shades and patterns so a broader range of fabrics were evidently considered acceptable.
It's my understanding (others please correct if wrong) that by the mid-60's sack and frock coats were pretty much worn at the pleasure of the individual for ordinary daywear. The sack coat was considered less formal, and therefore would be more appropriate for an office worker, clerk, or mechanic, while the frock would be more fitting for a proprietor, manager, doctor, or official. However, most middle-class men probably owned both styles. Mr Mayor might wear a sack coat while travelling, and Mr Mechanic probably donned his one good black frock coat to go to church.
If you are interested in a custom pattern (for either sack or frock) please PM me. I use an 1866 tailoring book, so you'll either get a fit or an authentic
...Seriously I can't promise a perfect result, but it should get you closer "in the ballpark" than many of the commercial patterns available.
Hope this helps,