I'm not a fl350 expert and maybe its the video, but it seems like your tach is way off. doesn't seem like the
Engine is running that well. something is failing could be ignition or fuel system. It also seems to me like the belt is to long or maybe worn beyond the limits. It looks like a aftermarket belt. the driven clutch does not open is it because of the belt, or because the Engine is running poorly, or is the clutch bad? Do you have it in gear in the video? Could the drive clutch be bad and not engaging properly? make a line with a sharpe on the clutch face. the belt will wear it off and this will tell you how far the clutch is engaging.
Just my observations watching the video. I go through each system and ensure things are in spec, per the manual.