Also replace the gear box mounting bolts with new hardened bolts. Do not re-use the old bolts.Torque the bolts, don't just whale on them with an impact wrench. Now's a good time to inspect the rail mounting area for any rust or collapsing issues.

The centering bolt can be made using an 8 mm standard thread bolt. Grind the very tip of it into a cone and it will fit into the dimple on the gear box shaft.