Pull the brushes up high enough out of their brackets so that you can wedge the tension springs against one side of the brushes. This will suspend them up and away from the armature commutator plates. Slide the armature into the end plate past the suspended brushes. Once the armature's snout is seated in its bushing/bearing, release the springs, push the brushes down and then move the spring ends over the tops of the brushes to apply downward tension. Even this procedure is tricky and frustrating. But by the time you have raised the final brush, it will seem easier.
Marshall
ADDED: I replaced the brushes in a Model A starter last month using this method and it made life a LITTLE easier.