You will have to drill out the "special" rivets. Buy the new motor from GM and get the five rivets also. In fact, get a few extra because your going to bugger the first one up anyways until you get the correct tension down pat. You also need a large 22 inch pop rivet tool, a simple hand one won't do. When you drill out the rivets, have a piece of string cord looped around the rollers to catch the glass or it will fall and wedge in the track. It will take a few hours to do the job. Have GM parts print you out a schematic of the door parts to help guide you.
The problem is not the motor, but the screw assembly has worn out gears. That is why the motor tests out OK.