The worm shaft was the same size from '66-up and I think it was the same back to '62. The sector shaft changed in '73, worm stayed the same.
The power box worm shaft is smaller than the manual, so the manual worm has the greater number of splines between the two. So if there are two couplers to choose from, get the one with the greater number of splines.
The problem with the aftermarket couplers is there is no allowance for chassis flex like the OEM pot coupler has. The steering column will take a pounding, expect the bearings or bushings in the column to eventually fail.