Generally hardy to zone 5, they (the boxwoods) would be marginally hardy in a container in zone 6 w/o some protection. Roots are the least hardy of tissues you'd be concerned about over winter. Siting them on the ground on the north side of a heated building out of wind and sun along with mulching would probably insure survival of expected low root temperatures. Burying the container to the rim and mulching would be even better. The ferns will appreciate the same treatment or, as you mentioned, being permanently planted in a shady spot.

Al