I got a Northern Sampler from Sandhill one year and at least one variety did well for me in Zone 6 "Frazier White"
http://www.sandhillpreservation.com/...html#varieties
I got better results with an Asian variety called Koimo (pale yellow flesh, reddish purple skin, excellent sweet baked!), this one is semi-bush/early and the sweet potatoes grow straight down in a clump. I imagine this kind will do fine in a 55/gal or grow bag. It roots a bit while spreading around but kept pulling it on a fence at first then later just let it be.
More info on growing sweet potato on Northern areas
http://www.mofga.org/Publications/Ma...1/Default.aspx