219
Being somewhat familiar with the area, I would recommend using the C&O/GAP to Garrett. At Garrett, cross 653 and take Garrett Shortcut road up to 219. Garret Shortcut will be a steep hill followed by rolling hills, moderate traffic, fair shoulder. Follow berlin plank rd into somerset and take 601 to Johnstown. I'm not sure of the shoulder, but I've driven it, and remember it being fairly empty between US 30 and Johnstown. It will be busy near somerset though, but 219 between somerset and johnstown is an expressway, not ridable.
I would take advantage of of the C&O/GAP, and your ride may be more miles, but may be faster due to fewer steep hills and intersections. Less glass, nails, unmarked roads, and all of that other stuff to make a ride unplesant too. The GAP between cumberland and garrett is very impressive, and is the only way to cross the mountains. There are a few shortcut roads off the C&O also, to cut out some the river bends.
I would expect this ride to take 3-4 days at about 80-100 mi/day.