68 GMC Steering Box Steering Shaft question

[SangerV]

Hey all I need help with my 68 GMC K2500.

Had to have the steering box rebuilt, and when installing it I need about 1/2"
adjustment in the length of the steering shaft. I think collapsible shafts were mandated in 67 and looks like the lower 1/2 of the shaft is a tube and the upper 1/2 a solid shaft but it won't slide upward even with plastic hammering and penetrating oil. I see two holes drilled in the tube part with white in them is this a shear pin or what ? Any suggestions to get some adjustment ?

Thanks