I thought '73 still had the solid linkage w/o the nylon bushings? I know my 75 has the dreaded nylon bushings, but IIRC my 73 had the older style linkage before they went to the nylon bushings (unless someone swapped in a later wiper transmission setup that uses the bushings...). I'd pull the wiper motor first, could be the nut holding the crank arm has indeed backed away and the spline on the wiper transmission arm is no longer in contact with the spline on the motor shaft. Nothing to lose by trying that first.