when I did my 49, the passenger arm was slightly too long and didnt let the wiper go all the way down or all the way up. this was my fix
http://67-72chevytrucks.com/vboard/s...&postcount=637
make sure your linkage isnt binding or going over center on the transmissions (the spindle that the wiper attaches to). the instructions tell you to bend the linakges, but in my opinion that wastes the torque of the motor on bends, keeping it straight is the goal for me.