the SM319 was the OD variation of a 318, still referred to as the 318 by most folks - the early, non syncro 3 speed was a Muncie, and the later [post 66 or 67 or 68] all syncro 3 speed is a Saginaw [SM326], the OD for either was a Borg Warner add-on tail piece ....
OD rebuild info .... all 3 trans'
listed hereBill