why does the over center spring have to be removed

The over center spring will cause the pedal not to return but like 67 Dart said it shouldn't effect the disengagement. Each one of the rods have a oblong shaped holes which add up to the pedal free play. Adjust the linkage so that the throw out bearing just barely clears the pressure plate fingers. The fingers don't require much movement to release the disc. You may have mismatched components if it doesn't release somewhere in the middle of the pedal stroke when it is adjusted properly. tmm