New Pitman Arm Help Please

Hi guys, I installed a new pitman arm purchased from Rock Auto. Seems the same as the one removed however I can't seem to tighten enough on the steering box to show any tread at the bottom of the stud. I don't want to damange the splines or overtighten, just wondering if anyone has run into this issue and what the solution is?
It's on my dad's 65 Barracuda.