We tried direct driving 2 snow blower motors for our extension arm as well, unfortunately we had the same issue. The motor was very under powered and didn't lift the arm well and when it did the motion was very quick, not quite controllable. We eventually scrapped the idea of using these motors and are now using a single CIM motor with a gearbox articulate.
Our arm was a box of 1in-1in 80-20 extrusion about 16 inches long and 14 inches wide, If you are sticking with the snow blowers make sure the arm is pretty light maybe box extrusion or even PVC.
http://team-paragon.org/wp-content/u...Motors-571.pdf
This link has a list of motors and power ratings, the snow blowers are one of the weaker ones on the chart