Substitute a 5/8" grade-8 NF bolt, or similar sized shoulder bolt of the appropriate length (measure the shock eye diameter and required offset from the frame to verify the needed bolt diameter and length), and then fabricate a spacer to fit between the shock absorber eye and frame rail. Anchor the bolt to the frame using hardened washers and torque-prevailing grade-8 nuts. Carefully inspect around the shock pin hole through the frame for starburst and sub-concentric cracks. Cracks demand repair before continuing, since moving the shock mount further away from the frame face will grossly increase the leverage applied to the rail via the added pin length, exacerbating tin-canning of the rail and development/propagation of cracks. Hence, you will need to fabricate brackets that support the free (outer) end of the upper shock pins to diminish the added stress focused onto the rail faces. A rear upper shock support bracket is commercially available for square body trucks, but may not provide suitable axial support for your particular application.
Edit:
Of course, there is always a caveat. Substituting a straight bolt for the shouldered shock pin (your image) as suggested makes it difficult to control preload on the rubber (or urethane) shock eye bushings. The shoulder pin is designed to limit compression of the bushings as the outer nut seats against the shoulder of the pin. While the larger inner nut can be tightened to much higher torque, clamping the pin securely to the frame. The 'bolt' scenario is not an optimum solution. Controlling bushing preload is another reason the outer end of the 'bolt' needs to be supported, so the upper shock mount is firmly caged to the frame.
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