agarn, best to use a 46mm open end wrench to loosen the driveshaft clamp nut. Although I'm unfamiliar with your 300SE driveshaft, loosening the nut should allow the driveshaft to slide further so you're able to remove the flex disc. If you haven't done so already, be sure to match-mark the driveshaft sections and the transmission so it reassembles as-is.

__________________
Fred Hoelzle