Pivot shaft & poly bushings mystery

You should be able to put a long prybar between the inside of the LCA close to where the bushing is and the K frame and pry hard on the LCA to put it all in a bind to hold the pin still while you pop the nut with an impact gun. Once you get it tight enough to keep the pin from turning, you can torque it to spec if you want. I'd just tap it on down with the impact.