On our stock steering boxes, the pitman arm spline engagement is tapered. Should slip on at least 1/4 way before tapping it on further. My last install went on near all the way. In any case, clean the splines, male & female, then force something like a big screwdriver inbetween the split in the pitman arm to open it just a little to get it on.
But I've found that when I couldn't get the pitman arm back on right off, it was because I had not properly lined up the splines on the shaft and pitman arm to include the non-splined portions. So, if you're as mechanically inclined as I am, make sure you have perfect spline alignment before using a BFH.
