New Pitman Arm Help Please

Try to knock down any burrs or paint on the inside splines of the new pitman.

Take a stiff wire brush to it. Take a tiny file to the very ends of the splines. Look at the ends with a magnifying glass. Put some thread penetrating lube to the spines....

Are you basically trying to move it up 1/4” or so?

Like right now the nut is flush with the end of the pitman output shaft? And before there were threads showing?