I had to do this on same deck as yours. I had it together but was whining and sounded like metal was grinding every so often. It did not make this noise all the time. Not knowing about the phasing part when putting together I helped it using a ”dead blow” hammer, not good.
That’s when I decided to follow the instructions I saw somewhere and decided to slowly and carefully put together. Here’s what I did.
Clean off both spline shafts, then take a close look on each side and look for the spline that is spaced a little wider than the rest. Put a file mark on each side. Before mating to coupling, make sure that the splines do not have any burrs on it. Run a file through each spline and you can feel which one needs deburrin, do the same for the coupling side. Clean off real good, add grease to coupling and shaft prior to mating, should fall right in. In any case, if it’s not going in easily, do not use a hammer.