I have a couple of questions....did you get this worked out first of all??
Are you using the two speed crank arm or are you using a three speed crank arm? Makes a difference when converting from a two speed to a three speed motor. A two speed crank arm will NOT work, the motor will bind or slap around like you're talking about. Usually on a two speed crank arm it will say 2 SPEED and have a number stamped on it.
You'll also need to replace the two speed switch. On a two to three speed conversion you have to replace the link from the wiper motor to the driver's side pivot as well...at least on the 70-71 models....I've not had alot of experience myself with the later models.
Sorry I didn't see this thread earlier than this, but I get busy and don't get over here to cruise alot, but I'm trying to do better