brake drum dilemma

6 cylinders had 9" drums
8 cylinders had 10" x 1 3/4" -- that is what yours undoubtedly is --
the drum width is actually calculated by the width of the shoe....
the actual drum depth is greater since the shoe does not go from
bottom lip to upper lip....
A "Dart GT" is 90 + % 8 cylinder -- so should be 10" x 1 3/4"

(Canadians may have a different answer -- since some cars that went
north of the border, eh?, were equipped with lighter duty than here in the USA)....