My GUESSTIMATE is that you could substitute a 50 gram ball for each 2 ounce skein and have enough to complete the project. Remember, that is not absolute and will depend on what type of yarn, what gauge and whatever sizing changes you make.
The armhole bands are done a little differently. The stitches are picked up at the rate of 7 per block and the first row is knitted in STOCKINETTE on size 8/US (5 mm) needles. The rest of the armhole bands are knitted in 1 x 1 ribbing: 2 rows on size 8/US, 2 rows on size 7/US (4.5 mm), 2 rows on size 6/US (4.25 mm) and 2 rows on size 5/US (3.75 mm.)