Although the SB and Boxford are 99% the same note that the spindle nose on the SB is UNC angle while the Boxford is Whitworth, so the backplates don't swap. Most of the other parts you might want should be ok.
The star wheel on the front of the apron is the friction clutch for the feed (not screwcutting). It doesn't have a quick release but provided the apron oil reservoir isn't empty it should slip enough to allow you to hold the carriage back with the handwheel as your tool crashes into the shoulder you are cutting up to.
The screw in the middle of the star wheel is reverse thread so often broken off.