Help with rear shocks

[gizmoduck81]

Ok, I installed some KYB Monomax on the front of my 1963 GMC 1500 and they were such a great improvement I ordered some for the rear (KYB-565037). The problem I'm having is the upper mounting bolt/stud is 0.015 - 0.02" too large in diameter for the shock to slide on. The KYB has a metal sleeve, so it doesn't give at all. Is there a good way to remove material from the mounting stud? I feel like using course sandpaper would take forever and might not be uniform. It's welded on, so I can't remove it without cutting. I do have the studs from the front shock kit that I could use if I drill a new hole, but would rather use what's already there. There's just got to be a tool for this type of thing but since I don't know what it is it's pretty hard to google. I'm not entirely positive if this is even the original mount or if it's something a prior owner modified. Sorry if this is a super obvious/stupid question. This is my first restoration and I'm still learning. Thanks!