steering coupler replacement (how to)

First, I'd like to say "great site for beginners like me"
That being said, I'm putting a 71 Dart together, I put the motor in (360 magnum) over the weekend and the headers are hitting the coupling from the shaft to the ps pump just enough to knock the cover off. I'm going to re pull the column (had it completely rebuilt) and change the coupler out for a steering knuckle. On a 71 Dart, all factory, would it be the 3/4" 36 spline? or the 9/16" 26 spline.
Thanks.