If you have the rest of the gun disassembled, the solution may be easy. I think the pawl plunger and spring are interfering with the insertion of the hammer and pawl.
The Ruger Old Army uses the old SA revolver design and the pawl spring and plunger come in through the back of the cylinder frame. To install the hammer and pawl, the spring and plunger have to be removed. Then you insert the hammer and pawl, make sure the pawl is forward, insert the hammer screw, then insert the pawl plunger and spring.
Then you attach the grip frame, which holds in the pawl spring, and reassemble the pistol in the normal manner.
Jim
