Removing/changing power window motor

I have to remove/replace the power window motor on a '68 Imperial. It's located on the rear passenger 1/4 on a 2-door HT. Do I have to block the regulator mechanism before I unbolt the motor, or is the spring tension forcing the window up? I just don't want the mechanism to come crashing down on my hand while unbolting it. Is there something I need to clamp?

-Gregg B.